Chalk

drop it. keep the big bit. how much do you get back?

a stick of chalk breaks in three; keep the biggest. then decide how small a crumb can be

Break a stick at two random points and keep the longest of the three pieces: over many sticks the piece you keep averages 11/18 of the stick, about 61 percent. Real chalk gives back less, and the reason is a floor: no piece comes off shorter than about a centimetre. Every piece must then be at least that long, and the length to pay for it comes out of the biggest one, so raising the smallest allowed piece lowers the average of the largest. At a floor of one third all three pieces are equal. The maker of the video this came from predicted 0.61, dropped 260 sticks, measured 0.45, and found the crumb; the card puts that crumb under your finger.

Concepts: probability, expected-value, modelling.

An interactive you play with one thumb, on Wundr.