This blog post is a quick summary of the book “The Universe explained with a Cookie” written by Goeff Engelstein and illustrated by Michael Korfhage.

Geoff Engelstein

Michael Korfhage

I loved the visualizations in the book that go with the narrative. This book shows that by picking the right visuals, any reasonably difficult concept can easily be conveyed in a way that most of us can understand.

The book uses “cookie” or a topic that is “cookie-adjacent” to illustrate a variety of ideas in science. This connection between something that we are all familiar and something that we might be unfamiliar, creates visual cues in our mind that readers will remember for a very long time.

The first connection is between the number of atoms in a cookie and number of stars in the universe

We think there are about $10^{12}$ galaxies in the universe. And most galaxies have about $10^{12}$ stars. So there are around $10^{24}$ stars in the universe. One gram of the stuff that makes up most plants and animals has about $10^{23}$ atoms. A cookie is very similar in ingredients, and weighs about 16 grams. So a cookie has about $10^{24}$ atoms.

The number of atoms in a cookie is about the same as the number of stars in the universe.

Dark Matter Explained with Flour

Similar to the force exerted by Glutens, that hold dough particles together, it is the dark matter that holds our universe together. To highlight the above theory, the author gives a quick recap of four kinds of forces

We know of only four fundamental forces in the universe: gravity, electromagnetism, the strong force, and the weak force. Basically:

Gravity is the force of attraction between all objects in the universe. It makes planets orbit the sun and cookies fall when you drop them.

Electromagnetism is the force between positive and negative charges. It is the force behind electricity and light, and it binds atoms and molecules.

The strong force holds protons and neutrons together in atomic nuclei.

The weak force is involved in radiation and other subatomic processes.

Out of the above four forces, it is the gravity that is talked about when we refer to forces between planets and galaxies. Galaxies are all rotating and hence anything that rotates, a force must be pulling the stars toward the center to keep them rotating, rather than flying off into space. Measurements showed that the stars on the edges of the galaxies are moving way faster than they should. Galaxies should be flying apart as gravity forces decreases as one moves away from the center. But they aren’t

The simplest explanation is that space contains a type of matter that we don’t know about and is very difficult to detect, since we haven’t seen it before. It must be invisible and not interact with light at all, since light just passes right through it. So the mysterious substance was named dark matter. Dark matter was providing extra gravitational gluten to hold the galaxy together, allowing the outer-rim stars to zip along.

Fusion Explained with Sugar

How do we get energy from Sugar ?

All types of energy are related and can be converted into one another. So let’s start by looking at the key source of energy for humans and animals, and one of our cookie ingredients—sugar. There are actually many types of sugar. The one we cook with is called sucrose and is made up of two smaller sugar molecules joined together—fructose and glucose.

When we eat a cookie, our body breaks down the sucrose and other carbohydrates, converting them to a form the body can use. They get stored in glycogen in muscles and the liver for short-term use, and in triglycerides in fatty tissues for long-term storage.

Blood carries glucose to the cells in your body. On average, about 4 grams of sugar are in your bloodstream—basically a teaspoon. A teaspoon of sugar helps the energy go round.

The sugar is absorbed by cells and transported to the mitochondria inside them that produce energy. The glucose goes through a series of chemical reactions and combines with oxygen, which releases the energy that keeps living things alive.

How much energy do we get from a cookie? Fortunately, there’s a handy guide right on the side of the box—the calories. Calories measure how much energy a food can give to your body. There are other measures of energy besides calories—horsepower, BTUs, kilowatt-hours, joules, and many more. But they all measure the same thing, just as you can measure distance with inches, feet, centimeters, miles, furlongs, or light-years. You can use any unit you want, but measuring the distance from New York to Boston in inches or light-years isn’t exactly practical.

My in-depth research for this book involved reading nutrition labels for a lot of different types of cookies. And eating them, of course. For science. So I can tell you that the average cookie is about 150 calories. When that energy is released, it is enough to power a standard household 10-watt LED bulb for about seventeen hours.

How does the energy get in to the cookie in the first place? Photosynthesis. Sunlight provides the vast majority of the energy that drives our planet, yet we only receive a small piece of sun’s power. Sun produces this vast energy from the same force that holds protons and neutrons together - strong force. However this strong force only acts in very short distances.

When you bring two nuclei together, at first, they want to push each other away. The nuclei are positively charged, and when they get very close together, the charges repel each other, the same way you feel a force when you try to push together two north ends of magnets. Both the magnets and nuclei pushing apart are from the electromagnetic force.

But if you push them close enough together, the nuclei will want to bind together, thanks to the strong force. The strong force is what holds protons and neutrons together in the nucleus. And while it is stronger than the electromagnetic force, it only works at very short distances. So it doesn’t help pull the nuclei together until they are very, very close. You need to supply a lot of energy to overcome the electromagnetic force and push the nuclei close enough together for the strong force to kick in. But once you do, they release energy—a lot of energy. This is called nuclear fusion. And the energy released is way more than what you need to use to move the nuclei close enough for them to fuse. So you put a lot of energy in, but you get even more out.

Our sun, like all stars, is made up mostly of these lighter nuclei, giving them a lot of fuel to power nuclear fusion. But how does the sun overcome the energy barrier to force nuclei close enough together for fusion to take place? Gravity.

The sun has about 330,000 times as much mass as the earth. (“Mass” is a measure of how much stuff something is made of.) Gravity pulls all that together, squeezing it at the core and raising the temperature. The high temperature and pressure make it possible for energetic nuclei zipping around to smash into one another fast enough to overcome the electromagnetic repulsion, fuse together, and release energy. This energy helps increase the temperature inside the star even more, leading to a chain fusion reaction.

Have we able to replicate what happens in the Sun on Earth ? Not really. What about converting mass in to energy ?

If we could convert mass directly to energy, we would release even more than we get from a fusion reaction.

Unfortunately, the only way we know of to completely liberate all the energy contained in matter is to collide it with antimatter. Every particle has an antiparticle partner. If the two meet, they annihilate into a burst of energy. Antimatter is produced in high-energy particle collisions, but it typically doesn’t last long and is hard to store. You can’t keep it in a container since it will annihilate when it hits the edge of the container. As with fusion plasma, we need to use magnetic fields in vacuums to contain antimatter and keep it from regular matter so we can study it

The following summarizes the elusive world when we discover anti-matter

Atomic Structure explained with Salt and Baking Soda

Using “Baking Soda” as an example, the author explains concept of bond between two elements in a molecule. Sodium bicarbonate, also called as baking soda has a specific atomic structure with single bonds and double bonds. Bond in layman terms means two atoms are sharing a common electron. The way atoms are attached to each other might give rise to different elements, for example, diamond and graphite have the same atoms but difference lies in how they are connected.

Periodic table is the first attempt to organize elements in the world. They are organized in rows and columns so that columns capture the number of protons or number of electrons in them

The last column in the periodic table has a special property; the elements don’t react with any other elements. These are called the noble gases, and they are basically inert. Using the analogy of happy atoms, the author explains that

These element numbers—2, 10, 18, 36, and 54—make for happy atoms. Those numbers are what all atoms want to be. That’s why they don’t react with other elements. They don’t want to share electrons since they have the perfect amount.

Chlorine, for example, is 17. If it had just one more electron, it would have 18, one of our happy numbers. Sodium, on the other hand, has 11. If it had just one less electron, it would have 10, another happy number.

Sodium has one electron it wants to get rid of, and chlorine would really like one. How about if they get together and sodium lets chlorine borrow an electron? That’s what a chemical bond is—atoms sharing electrons. And that’s how our salt molecule, NaCl, gets made.

For the structure of the atom itself, there were many theories that were proposed starting from plum pudding model

In the 1890s, British physicist J. J. Thomson showed that atoms included a particle that carried a negative electric charge that was much, much lighter than the full atom. Also, he found that the particles were the same regardless of which element they came from. He called them corpuscles, which, honestly, is not the best name. Fortunately, other scientists adopted the term electrons.

With the discovery of the electron, Thomson proposed a model of the atom called the plum pudding model. Plum pudding is a traditional British dessert, made with plums, raisins, and other fruit—a relative of the American fruitcake.

He suggested that atoms were composed of a ball of positive stuff (the cake) with electrons floating around inside (the raisins).

I prefer to think of it as an M&M cookie, with the electrons represented as the candy bits.

The next theoretical model was proposed by Ernest Marsden and Hans Geiger.

The atom couldn’t be a low-density cake taking up the full volume of the atom. All the cake material was squished into a very, very tiny area in the middle of the atom. The plum pudding model was dead.

The classic picture on the right side of the electron “orbiting” the atom’s nucleus like planets orbit the sun is actually very misleading. Electrons don’t really move in circles. They flit from place to place, creating an electron cloud around the atom. This is due to quantum mechanics—the physics of very tiny things—which we will dive into in a later chapter. So while this orbiting picture is a better model than the plum pudding model, it’s still far from the perfect picture. Here’s a slightly better one:

The scale of atoms—the size of the nucleus versus the size of the atom overall when you include the electron cloud—is not really comprehended by most people. The standard picture of an atom does not give you a clue as to how small the nucleus is.

As a way to picture this, let’s blow up an atom until the nucleus is the size of a chocolate chip. How far away will the electron cloud be? Any guesses?

Let’s put our chocolate chip nucleus on the fifty-yard line of a football field—right smack in the middle. The electron cloud, which defines the size of the overall atom, will be at the end zones.Sometimes you’ll hear people say that atoms are mostly empty space—this is why. Everything between the chocolate chip at the fifty-yard line and the electron cloud that starts around the goal lines is empty. That’s a lot of empty space.

The author uses the analogy of “blind cookie exchange” game to create visual cues for the reader to understand various concepts relating to quarks.

Exchanging cookies for holidays and other celebrations is a fun tradition and a great way to discover new cookies and great recipes. Let’s say you have a friend who throws a “blind cookie exchange.” Everyone brings their cookies wrapped up nicely in little bags. Unfortunately, we can’t see inside them, and the etiquette at this party doesn’t allow us to peek. How can we find out what cookies we’ve gotten?

Well, if you’re an experimental physicist, your first instinct is to fire two of these cookie bags at each other really, really fast and then sift through the debris to see if you can figure out what the cookies were.

Physicists observed that when two protons are smashed, there are other additional particles that are come out of the experiment and nothing happens to the protons.

The way to imagine elementary particles such as protons and neutrons are to think of them as a bag

Each bag can have the following contents

Imagine that you have one such bag from the party. Quark theory says

Every bag must have three cookies in it.

The three cookies must be three different colors (one red, one blue, and one green).

Bags act differently based on the three flavors in the bag, not their colors.

Using this analogy, the author introduces quark theory saying

In quark theory, the “bag” represents the particle, like a proton or neutron. The cookies inside the bags are called quarks.

Like the cookies, quarks can come in multiple flavors. We currently know of only six, and they come in three pairs, with each successive pair being heavier. The two lightest are called Up and Down. The middle-weight quarks are called Strange and Charm, and finally the two heaviest quarks we are aware of are called Top and Bottom. Originally those were named Truth and Beauty, but physicists decided they had already gone a little too far with Strange and Charm and wanted to make their theory sound more serious again.

In our cookie-bag model, Up, Down, Strange, Charm, Top, and Bottom are the different flavors of the cookies. And each of these six flavors also comes in three colors—red, blue, and green.

A proton is made of two Up quarks and a Down quark (UUD). A neutron is one Up and two Downs (UDD). But those aren’t the only combinations, of course. Each unique combination of quarks results in a particle with different characteristics, a recipe if you will.

A further illustration of related concepts takes you to the Standard model of all particles

Besides quarks, we are aware of only a handful of other elementary particles. Electrons are the simplest type of a particle, called leptons. Like quarks, which come in three “generations” (Up/Down, Strange/Charm, Top/Bottom), there are three generations of leptons—the electron, the muon, and the tauon. And each of these three has a partner called a neutrino. As far as we know, leptons and neutrinos are not made up of smaller particles.

The names and specifics of leptons and neutrinos aren’t important. I list them just to give you an idea that there really aren’t that many fundamental particles and that they are organized.

These twelve particles—six quarks, three leptons, and three neutrinos—form the basis of what is called the Standard Model.

When we dip a cookie in to the milk, you can see the waves formed on the milk. So fundamentally it looks like milk and cookie are different in nature. The author gives a crash course on waves, their properties and then goes on to describe light as being both wave and particle.

Light was not just a wave—it also acted like a particle.Scientists also were learning that particles weren’t always what they thought. When they were first discovered, electrons seemed like particles. They had mass, for example. Yet experiments and theories were showing that electrons could also act like waves. Electrons could create interference patterns just like waves! Milk could be cookies, and cookies could be milk.

The theory and equations of quantum mechanics were developed to explain how things could be both milk and cookies—both waves and particles.

One of the consequences of everything being both milk and cookies is that we can’t say precisely where an electron (or other tiny object) is or what energy it has. The possible locations are spread out across space. This is partially related to the wave nature of objects. You can’t say precisely where a wave is.

Quantum mechanics lets you calculate the probability of finding the particle in a particular spot or with a particular energy. It does not say exactly where the particle is or how fast it is moving.

Evolution explained with Butter and a Baking competition

The author uses a hypothetical baking competition to drive home the key ideas behind evolution. I think the following serves as a good visual cue about the competition

Genetic Engineering Explained with an Egg

How does a single egg become a chicken ? How are the instructions in a cell packaged? Using four different types of cookies, the author shows a way to form words that can be used to escape from a hypothetical hell.

This analogy is used to illustrate the way instructions are laid out in a egg

ESCAPE MESSAGE = THE CHICKEN WORDS = PROTEINS LETTERS = AMINO ACIDS

This chapter talks about the machinery used in taking the message that lies in a cell to actually creating a species.

I want to create a cookie with a nice design on it, done in frosting. But I’m super lazy, and I don’t want to put the frosting on myself. Instead, I’ve invented a tiny little machine that can put frosting right in its area.

I’ve got a blank cookie, and I’ve scattered thousands of these little frosting machines across the face of the cookie, like pixels on a television screen. I can give each of these little dot machines instructions on when to make frosting and when not to.

But because I’m lazy, every single machine has to have the same instructions. I only want to write one set of instructions, not thousands!

How can I set this up to make an intricate pattern?

Before we solve this problem, I will tip my hand and confirm what you probably already suspect. This self-frosting-cookie problem is analogous to how an embryo develops. Embryos start out as one cell, but quickly they duplicate into a mass of thousands of cells. Somehow the cells need to decide if they are going to make a bone or a muscle or skin or an eye. But every cell has the same instructions—they all share the same DNA.

Using a set of nice easy to remember visuals, the author talks about the way cells grow in to various organs in a body.

Uncertainty Explained with 3/4 cup of packed brown sugar

This chapter is all about ways measurement error that inevitably crops up as soon as one starts to measure something

Another possible source of error, both in measuring the ingredients for our gingerbread house and testing its strength is the measurement itself. When you measure out ¾ cup of brown sugar, how accurate is it really?

There are several possible sources of error: You could not fill up your cup exactly level—perhaps the brown sugar is a little below the surface or is heaping up above. Or maybe it isn’t packed down as much as the recipe writer meant it to be.

However, it’s also possible that your measuring cup doesn’t hold precisely three-quarters of a cup. How do you know how accurate your measuring cups and spoons really are?

Devices used for measurement need to be tested against a known good standard. This is called calibration. Some equipment can be adjusted to make the readings match the standard. In other cases, like with measuring cups, they cannot be changed. If the error is small enough, they might still be okay to use, depending on the purpose. Or they might need to be taken out of service.

Calibration and testing extend well beyond the world of science. Measures used for commercial reasons also need to be tested. Government officials typically test supermarket scales and gas pumps annually to ensure they are accurate within accepted standards.

Thermodynamics explained with Baking and Ice cream Sandwich

The author uses “cookies being baked in the oven” to describe conduction.

The process of heat spreading through an area by collisions is called conduction. As the air molecules collide with the cookie, they transfer energy to its molecules. The air molecules lose energy in the process, cooling down. Then they hang around, waiting for hotter, higher-energy air molecules to hit them, to make them energetic again, so they once again can collide with the cookie

Convection through a pot of boiling water

and finally on how does daily life object, microwave work ?

Microwaves pass through the air without interacting with it. However, they can transfer energy directly to water molecules. Water molecules in food energetically vibrate when hit by microwaves and then bounce into the other molecules in the food, transferring the energy to them. All the conduction happens inside the food—the whole step of conducting heat from the heating elements to the food via the air is skipped.

The chapter ends with the way you can make icecream when you add salt to ice and suspend icecream mixture in to the container

Entropy explained with mixing

The author starts with a simple experiment of mixing flour, salt and baking soda and asks a question - can the particles be unmixed ?

Something fundamental has changed. You could theoretically unmix them—go through and separate the mixture back into a pile of flour, a pile of salt, and a pile of baking soda. But it would be difficult and take a lot of energy to do it. Mixing ingredients together is a one-way trip for all practical purposes.

One of the key things about Newton’s and Einstein’s laws of motion is that they are the same whether time runs forward or backward. If you took a movie of the planets moving around the sun and ran it backward, you would not be able to tell time had been reversed.

But this simple example of mixing our powder ingredients together shows a different truth. There is an “arrow of time.” Explaining this arrow has led science to some deep and interesting places.

The author does a cookie flipping experiment to show that

So is it possible that after you thoroughly mix the ingredients you find all the salt in one spot? Yes. But it’s incredibly unlikely. If you flip one hundred cookies, it takes 1029 years’ worth of shuffles to maybe return to its starting position, so your bowl of ingredients will take many, many times longer.

The objects we deal with in our world, and the planets and stars, are made up of astronomically many more particles. There are about ten thousand grains of salt in a teaspoon and about 1022 molecules of NaCl. So there is going to be basically no “ripple” in their random behavior. Randomness is still behind the scenes, but the huge number of particles averages it out.

Going back to Chapter Five about milk and cookies, that’s why individual molecules can follow the weird rules of quantum mechanics, but a teaspoon of salt is perfectly well-behaved.

Chaos explained with Vanilla

The author starts with a wonderful story of how a 12 year old enslaved person in Reunion, near Madagascar was instrumental in making Madagascar the top producer of vanilla in the world. The story is mainly to highlight the importance of making observations and drawing causal inferences. However not every system is predictable.

Chaos the way scientists think of it has two features:

It is deterministic—if you repeat the same steps again with the same starting point, you end up in the same place.

It is unpredictable—small changes in the start position lead to big changes in the end position, and often it can be difficult to tell what is going to happen in the future.

Along the way, the author mentions about Lorenz waterwheel, a perfect demonstration of chaotic systems

The author does a great job of explaining complexity theory using a little game using cookie cutters. He also poses several simple to describe problems that are classified as NP-Complete(it is easy to tell if a solution is valid but really hard to find the best one)

Using cookies, the author describes how cryptography works

Here’s a pile of 7,493 cookies. Can you arrange them so that they form a rectangle, where both sides are two or more cookies long? Every time I ask you, I put in a different number of cookies, of course. 7,493 is just an example. For example, if I have a pile of twenty-one cookies, I can arrange them like this:

But if I give you a pile of twenty-three cookies, you can’t arrange them in a rectangle. If you make a six-by-four rectangle, there will be one spot empty, and if you try a two-by-eleven rectangle, there will be one cookie left over.

If you can’t put the cookies into a rectangle, you have a prime number of cookies. If you can, it’s called a composite number of cookies. Sometimes you can make many rectangles—like if I give you twenty-four cookies, you can make a two-by-twelve, three-by-eight, or four-by-six rectangle. But twenty-five cookies can only be made into a five-by-five grid. The number of cookies on each side of the rectangle is called a factor of the number. So two, twelve, three, eight, four, and six are all factors of twenty-four. Five is the only factor of twenty-five.

If there’s only one way to make the rectangle, it’s going to be harder to figure out if the number of cookies you have is prime or not. Security and encryption on the internet are based on this idea. The root of the security is a really big number that one computer creates, that it knows is not prime, but has only two factors.

The computer on the receiving end of the message knows one of the sides of the rectangle (one of the factors), so it is easy for it to calculate the other factor, which is the secret value the first computer is trying to hide. But if a third person gets the message, it will take them a very long time to figure out how to arrange those cookies into a rectangle.

For example, it may not be obvious to you that 7,493 cookies can be arranged into a rectangle that is 127 x 59 cookies. But if you know that one side is fifty-nine cookies long, it will be very easy for you to figure out that the other side is 127.

This is a bit of an oversimplification of encryption and security, but it should give you the general idea of how it works.

The key point is that it is very important to the security of the internet that no one can figure out how to easily turn really big numbers of cookies into rectangles. If that complex problem becomes quickly solvable, that will cause big problems.

Unfortunately, researchers have figured out how to do that—theoretically at least—using something called a quantum computer.

The author tries to delve in to the boundary of the cookie to explain the fractal nature of the thing. Even though the size is fixed, the boundary length is infinite

One of the simplest shapes that, like an oatmeal raisin cookie, has a finite area but an infinite border is called the Koch snowflake, discovered in 1904 by Swedish mathematician Helge von Koch.

Exoplanets explained with a nice golden brown color

The author describes the way humans see “color”(a color is light at a specific energy).

We don’t see the “color” of light directly. Receptors in our eyes respond to red, blue, and green light, and our brain combines these to create colors, like the golden brown of our cookie.

How is light measured ?

Light is a little packet of electromagnetic fields bouncing up and down. While the light packet itself always moves forward at a constant speed—the speed of light, of course—it can vibrate at any frequency. And this frequency defines the color. There are two choices on how to describe a light wave. One is to measure the frequency, how many times per second the wave wiggles up and down. Another is the wavelength, which is the distance between the peaks of the waves. Since the speed of light is constant, either the wavelength or frequency is enough to tell you about the color of the light. For visible light, we most often use wavelength, measured in “nanometers” (nm). A nanometer is a billionth of a meter, so quite small. Blue light, for example, is about 500 nm, and red light is about 700 nm.

What is a visible spectrum ?

The visible spectrum contains an infinite number of colors, but Newton broke it into seven—red, orange, yellow, green, blue, indigo, and violet, mainly because he thought the number seven had special significance. But there’s no reason at all that it needs to be seven. In fact, Newton at first decided it was six colors, but he threw in indigo at the last minute because he wanted seven.

The spectrum doesn’t end with the visible portion. If you move past red, you get to infrared, microwaves, and then radio waves, as the wavelengths get longer and longer. If you go past the violet end, the wavelengths get shorter and shorter, and you get ultraviolet light, x-rays, and gamma rays.

Scientists noticed that different elements made flames of different colors when they were burning. Our old pal sodium, from salt, burns orange. Potassium burns pink, and copper green. Some elements, like neon, glowed when electricity was passed through them.

When they looked at the light spectra of these flames, they each showed a different pattern of lines. These lines form a fingerprint of the atom.

Scientists also discovered that elements only absorbed light that was the same frequency as what they emitted. If light passes through a cloud of mercury, lines appear at the same wavelengths.

When they went back and looked at the light coming from the sun, the black lines matched up with lines from certain elements. The lines were telling us what the sun was made of. As light emerges from the sun, it passes through those elements, and certain colors get absorbed.

Scientists were able to match up the lines from the sun with the lines we could re-create here on earth and figure out the elements.

Ever since we discovered that the sun is a star, we have wondered if the stars also have planets orbiting them, like our solar system. Planets outside our solar system are called exoplanets. We were pretty sure there must be exoplanets, but we weren’t sure what percentage of stars had planets or what those planets looked like. How many of them were Earthlike? Was an Earth twin out there waiting to be discovered?

The author then provides a way that humans have used to discover “exo-planets”

Using these and other methods, we have now found over five thousand planets outside our solar system. We’ve only started to look and are still refining our techniques for finding them. The James Webb Space Telescope, which just became operational in 2022, has even allowed us to directly image larger planets directly

The Big Bang explained with Chocolate Chips

The last chapter explains big bang and ends with a visual that says that we don’t know 95% of the universe.

Epilogue

Flour and sugar led to understanding gravity and galactic structure, salt and baking soda to the tiniest particles and the quantum world. Vanilla, cookie cutters, and walking around an oatmeal raisin cookie brought us to fundamental truths about chaos, complexity, and how much we can ultimately predict. Mixing and baking led to understanding thermodynamics and entropy, which just might underlie everything.

And brown sugar and measuring spoons showed us how to be more accurate in our measurements and appreciate and deal with errors that always arise. Color taught us how to build a measuring stick for distances that boggle the imagination. Finally, eggs, butter, and a bit of cookie decorating showed us how self-replicating patterns could swirl, combine, divide, and evolve to develop the capability to understand all this.

Takeaway

This book serves a perfect primer for someone interested in physics. The book will make any person understand the basic concepts behind many physical phenomena and the fact that this book has rich visuals means, the concepts are going to stick. I found the book a delight to read. Also the way the concepts have been presented via rich appealing visuals is something that I have never seen in a Physics primer. Kudos to the author and the illustrator.