Toss a coin onto a grid of squares. The chance of it landing entirely inside one square (not crossing a line) depends on the size of the coin and the size of the squares — and the answer involves both length and area.
How to do it
On a sheet of paper, rule a grid of squares. Each square should be just a bit bigger than your coin. Toss the coin from a small height ten times — count how often it lands fully inside one square (not crossing any line).
If the square has side s and the coin has radius r, the coin lands fully inside one square only if its centre is at least r from every edge. That's a smaller square of side s − 2r — so the probability is ((s − 2r)/s)².