Yesterday, I introduced an old friend—the Möbius strip—in the context of topology, the study of objects independently of their size and specific shape. Since I’m currently away from teaching for the longest period in my life since 1999, I even assigned homework, which (typically) none of you did. I am *very* disappointed with you all. You’ll pay for your laziness when the final exam rolls around.

However, I’m a kind soul, so here are the “homework” solutions, with bonus mathy goodness thrown in. I used white typing paper, so unfortunately the pictures may not be as clear as would be ideal.

- “Make both types of Möbius strip, and show that you can’t turn one into the other without cutting or breaking the tape—just like your right and left hands aren’t equivalent.” The images below show construction of one type of Möbius strip, and then the completed left- and right-handed versions. I’ve colored one edge of the paper to guide, but I think you’ll be happier if you just do it yourself.
- “Naively, you might have thought that you could make a sphere by just taping the top and bottom of the paper together. Here’s the square representing that situation, which also is the universe of the classic video game
*Asteroids*(and its innumerable imitators). Figure out what shape this actually makes and leave your answer in the comments.” This shape is a little hard to make, so I decided to try a trick: cutting little squares out of the top and bottom. I think it will work better with a bit more experimentation, but the answer is…a torus, AKA donut shape. As you can see from the figure, that’s the same as making a cylinder, then connecting the top of the cylinder to the bottom. - “Now for a really fun one, which you won’t be able to do with paper, but perhaps can figure out anyway. Imagine taping the left and right sides together like a cylinder, but taping the top and bottom together like a Möbius strip. Draw the result (or locate a picture online that illustrates it), and leave your answer in the comments.” The shape you get is called a
*Klein bottle*, and the ideal geometrical version would involve passing paper through itself. However, my bloggy colleague Greg Gbur pointed out a way to construct this one with paper, if you don’t mind that it isn’t strictly a perfect version. My Klein bottle is shown below.

Again, I think you’ll be happier (and possibly more successful) making these shapes yourself. The fun in this stuff is never in looking at what others have made—go and mess around, and if you create something awesome-looking, send me a message and I’ll post it in a follow-up article!

**Bonus Features**

- As you probably expect if you follow science for any time, there’s almost nothing in math that doesn’t show up somewhere in physics or chemistry or biology. Greg Gbur wrote a post on the role Möbius strips play in optics, with extra topology!
- I am ashamed that I had totally missed the awesome work of mathematical artist Vi Hart until now, but I intend to catch up. As a starter, here’s her Möbius strip movie. Thanks to Peter Newbury for this link!

## 4 responses to “Topology With Paper and Scissors”

[…] math-related paper projects, so expect a post on that subject soon. (The earlier posts are here and here.) In fact, I may get the whole family involved making some mathematical shapes, since many of them […]

[…] I use, so I won’t attempt to liveblog my reading of the book. However, in the spirit of my earlier posts on Möbius and topology, I borrowed some scissors and raided the local paper supply to illustrate […]

Ahhhhh, that’s how you do it (use a stapler!!). I was confused! Thanks Dr. Francis

[…] shape I brought up in the previous paper-topology post was the Klein bottle, which we have to cheat a bit to construct. The template for you to make your […]