In A.D. 1858, the German mathematician Mobius (1790 ~ 1868) discovered that a piece of paper twisted by 180 was magically bonded at both ends.

Because ordinary paper has two sides (that is, a double-sided surface), one front and one back, and the two sides can be painted with different colors; And such a paper tape has only one side (that is, one side), and a bug can crawl all over it without crossing its edge!

We call this magical single-sided paper tape discovered by Mobius "Mobius tape".

Take a long piece of white paper, paint one side black, then turn one end over and stick it into a Mobius belt like the picture on the previous page. Now use scissors to cut along the center of the paper tape, as shown in the figure. You will be surprised to find that instead of splitting the paper tape in two, you cut a paper circle twice the length in the picture!