Intrigued by topology—the study of geometric properties that do not change when you stretch a shape— Bay Area physicist Cliff Stoll began making Klein bottles. Give a strip of paper a half-twist and tape the ends together— you get a Möbius loop, a one-sided shape with one edge. Make two Möbius loops and glue their edges together—you should get a Klein bottle. But this is impossible in our universe—you need four spatial dimensions! In our 3-D universe, every Klein bottle must have a self-intersection, where tubing crosses through a wall. See “Mathematics: Modern and Vintage” is on display post-security in Terminal 2 and online at: https://bit.ly/MathAtSFO This image was posted on October 25, 2021.