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When my kids were in 1st and 2nd grade, I realized that cutting snowflakes for Christmas decorations was a perfect way to learn about prime factorization. I took the opportunity to teach them, and this has paid dividends for their understanding of math for years. It made it easier for them to learn their multiplication tables, to learn to simplify fractions, and my daughter reports that when doing mental math she often factorizes the numbers first to make it easier. I wrote up my explanation as a lesson in case other parents or teachers want to try it with their own kids or class. It is intended for kids who have gotten comfortable with single digit multiplication with very small numbers, perfect for preparing for winter break in 2nd or 3rd grade. If you try it out, please let me know how it goes!
Let’s make a snowflake. I’m going to fold this paper in half 3 times, then cut it at an angle.

Can you guess how many points it will have when I unfold it? Before we unfold it, let’s think a little bit about it and make a prediction. To do that, I want you to think about how many layers of paper we are making as we fold it. What happens to the number of layers of paper when we fold it in half?
First we go from 1 to 2, then 2 to 4. When we fold a stack of paper, every layer of the paper gets folded, so every layer before the fold turns into two layers after the fold. That means every fold multiplies the number of layers by 2.
This diagram might help visualize it. Instead of the folds we’re actually making to cut the snowflake, suppose we just keep folding a piece of paper in half in the same direction each time, and then look at it from the end. From the end you can see the layers of paper wrap around, and you can see why it gets harder and harder to fold. It gets really thick! That’s a lot of layers.

That means when we fold it in half 3 times, we’ve made 2 × 2 × 2 = 8 layers. After we cut it, what do you expect to see? Make a guess, and then unfold it.

That’s surprising! 4 points instead of 8. How could we have predicted this before we unfolded? Notice that every point is made out of two layers of paper, one on each side of the point. That means that however many points we want, we should make double the number of layers before we cut. If we want 4 points, we should make 8 layers. If you go around the snowflake and count the number of sections that the folds split it up into, you can see there are 8 of them, just like we expected, it’s just that 8 layers of paper only make 4 points.

What if we want 6 points? All real snowflakes have 6 points, so if we want to make a snowflake look real, we better learn how to make a 6 pointed one.
(Why do they have 6 points? Because water molecules are shaped like a mickey mouse, and the ears of the mickey mouses stick to the chins of the mickey mouses, which makes hexagons when you get big groups of them. Make your hands into the “aloha” sign. Your fingers sticking out are like the ears of the mickey mouse, and the bottom of your palm is like its chin. Now stick each of your fingers to the bottom of somebody else’s hand. That’s how water molecules stick together when they freeze. If three of us get together with all 6 of our hands, you can make a full ring and see how the water molecules make hexagons.)
If we want a 6 pointed snowflake, that means we need 6 × 2 = 12 layers. 12 is more than 8, so maybe we fold it one more time? Let’s check. If you want, make a prediction for how many points it will have, or you can fold it, cut it, and see.

That didn’t work! That gave us 8 points, more than we wanted. If 3 folds gave us 4 points, and 4 folds gave us 8 points, how can we make 6 points? We kept folding, but the number of points skipped 6. Does that mean it’s impossible to make a 6 pointed star?
It isn’t impossible, but it’s impossible the way we’ve been doing it, and the reason it’s impossible is related to some of the deepest and most important ideas in all of math.
Because we want to end up with 6 points, we want to fold it to have twice that many layers, which is 12 layers. How can we break down the steps you need to make 12 layers? Do you know any numbers that multiply to 12?
We just found one, 6 × 2 = 12.
Do we know any numbers that multiply to 6? Yes, 2 × 3 = 6.
So putting that all together, 2 × 3 × 2 = 12.
Now think about what this means for the folds we have to do to make 12 layers. We start by making a fold that doubles the number of layers. That’s just the simple fold we’ve been doing. But then the next step is tricky. We have to make a fold that triples the number of layers. If we figure that out, we just have to fold it normally one more time, and we’ll have a 6 pointed star.
The fold to triple the number of layers is tricky, so let me show you how to do it. This is the hardest step, so you can get an adult to help you if you need it. You can’t do it by matching up sides exactly the way we normally fold things, you have to eyeball it and estimate. First you find the middle of the folded side by matching up the corners, and pinch at the fold, but don’t fully crease it. Instead take the side you’re folding over, and turn it so that it’s pointing up and to the right. You want the paper you can see on the layer below to be the same size near the point as the part you’ve folded over.

That’s really difficult to do. It would be nice if we could find some way of making a 6 pointed star that didn’t use that weird 3 layer fold. Is there any other way we can fold it? Let’s go back to our goal. We need to make something with 12 layers. Do you know anything else that multiplies to 12?
How about 4 × 3 = 12? We know how to make something with 4 layers, just fold it in half twice. So the folds we’d need to do are 2 × 2 × 3, but then we’re still left with that annoying triple fold at the end. Are we really stuck? Let’s write down every single option we have to make sure we haven’t missed anything.
2 × 6 = 12
4 × 3 = 12
3 × 4 = 12
6 × 2 = 12
4 and 6 are still too big to fold in one step, so let’s break them down further. 4 = 2 × 2, and 6 = 2 × 3, so we can replace those numbers.
2 × 6 = 2 × 2 × 3 = 12
4 × 3 = 2 × 2 × 3 = 12
3 × 4 = 3 × 2 × 2 = 12
6 × 2 = 2 × 3 × 2 = 12
Now look at that. Do you notice the pattern? Whenever we break down the folds we’d need to make to make 12 layers, we always end up with two folds where we fold it in half, and one fold where we fold it in thirds. It doesn’t matter the order in which we fold them, you always have to have two 2s, and one 3.
This pattern is called the “prime factorization” of 12. Don’t worry about what “factorization” means yet, but think about this word “prime.” You might have learned about the primary colors in art class. All the colors that people can see can be made out of mixtures of those primary colors, red, yellow, and blue for paint, or red, green, and blue for light. “Prime” numbers are the same way. All numbers are either prime, or can be made by multiplying prime numbers together. Every number is a mixture of prime numbers in the same way that every color is a mixture of primary colors.
Even crazier, there is only one way to make any number by multiplying primes, if you ignore what order they’re in. And because there’s only one way to make a given number, that makes it really really easy to find. Just start by breaking down the number any way you can remember. Then check each of the numbers you get, and see if they can be broken down any further.
12 is 6 times 2. 2 can’t be broken down any more. 6 can. 6 is 2 times 3. 2 and 3 can’t be broken down, so we’re done. 12 = 2 × 2 × 3.

It doesn’t matter what order you break down the number, and it doesn’t matter what order you fold the paper. You’ll always end up with the same result.
Suppose we want to make a 5 pointed snowflake, like an ordinary star. Can we use what we’ve learned to figure out how? Let’s figure out the prime factorization of 10.
10 is 5 times 2. 2 can’t be broken down anymore. Can 5? Nope. 5 is also a prime number, just like 2 and 3.

So that’s it. If we want to cut a 5 pointed snowflake, we have to figure out how to fold something 5 ways. Ich. That sounds even harder than folding something 3 ways.
What about 16? Let’s try to break it down.

Again, no matter what order we break it down, we get the same result. 2 × 2 × 2 × 2. If we want to make a star with 8 points, or 16 layers, we always have to fold it in half 4 times.
So that raises the question, how hard can making a snowflake with a certain number of points get? Is 5 the hardest fold we’d ever have to do?
No, in fact. There are an infinite number of prime numbers, and they just keep getting bigger. I’ll write out the first few below.
2, 3, 5, 7, 11, 13, 17, 19, 23, …
So if you want to make a 7 pointed star, now we know it’s going to be really hard. First fold the paper in half, then somehow figure out how to fold it into equal sections of 7. I’ll stick to 6 points! That’s hard enough.
The fact that every number can be broken down into prime numbers multiplied together, and you always get the same result no matter how you break it down, is so important to math that it is called the Fundamental Theorem of Arithmetic. It is so important because it shows up everywhere, even in cutting snowflakes.
Happy cutting, and if you need some ideas for patterns, Veritasium has a wonderful video about where the shapes of snowflakes come from.

If all that was easy, here’s one more thing. When we write out the prime factorization of something like 16, writing out 2 × 2 × 2 × 2 is a lot of work, so instead we just write that as 2⁴, where the 4 counts how many 2s are multiplied together. Using this way of writing it, we can write out the prime factorizations of the first 20 numbers in a really short way. This is a great thing to practice, either to memorize them, or to come up with them quickly. That little number ⁴ is pronounced “to the fourth” when you say it out loud.






























































































































