From Futility Closet:

In the top figure, one coin rolls around another coin of equal size.

In the bottom figure, the same coin rolls along a straight line. In each case the rolling coin has made one complete rotation. But the red arc at the top is half the length of the red line at the bottom. Why?

http://feeds.boingboing.net/~r/boingboing/iBag/~3/oTiSkEGt2xQ/the-coin-paradox.html