Futility Closet posted a puzzle that might make your eyes glaze over, or could spur some of you to compete over who has the best, clearest explanation.
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?
I look forward to any explanation you may have, and later I’ll add some I found elsewhere. -via Boing Boing
(Image credit: Lymantria)