CH 45: Mobius Strip: Assembly Required
The Mobius strip is a classic example of a physical paradox. It is a closed surface that has only one side. The result is a surface that in mathematics is called non-orientable. If an ant was placed on a Mobius strip and walked all the way around the strip, the ant will have traversed both sides of the strip of paper without ever having to cross an edge of the paper to get to the other side.
You can make your own Mobius strip by taking a thin strip of paper, twisting it once and then taping the two ends together and voila, a mathematical paradox.
Ant Traversal Trickery