Darts

throw darts at the square. the share in the circle finds π.

darts at a round board on a square of cork; four times the share on the board closes in on pi, but only if you stop aiming

The board is a circle drawn exactly inside the square of cork, so its area is pi/4 of the square. Throw darts truly at random over the square and that share of them lands on the board, so 4 x (on the board / thrown) estimates pi. Aim, and the count says whatever your arm chose. Chance is slow: the error shrinks like 1/sqrt(n), so each new digit takes about a hundred times the darts. (The blind darts here are perfectly uniform; a real blindfolded thrower is not, which is why it has to be chance and not a person.) This is the Monte Carlo method: estimating something by random sampling.

Concepts: probability, pi.

An interactive you play with one thumb, on Wundr.