一张纸，只有一面的环

Take a strip of paper. 拿一条长长的纸条。 Twist it once, then glue the two ends. 把它拧一下，再把头尾粘起来。 You get a ring with only one side. 你就得到一个只有一面的环。 A small bug can walk on it without crossing an edge. 小虫爬上去，不用翻边就能走完全程。 This is called a Mobius strip. 它的名字，叫莫比乌斯带。 It looks simple, but it is very strange. 看着简单，其实奇怪得很。 Some roads use this crazy idea. 有的公路设计，就借用了它的点子。