Weigh

nine coins, one is heavy. two weighings to find it.

put coins on a two-pan balance and find the heavy one in two weighings

A balance only ever gives one of three answers: left, right, or level. So each weighing cuts the suspects to a third rather than a half: three against three leaves three, one against one leaves one, and two weighings are exactly enough for nine coins.

Concepts: logic, information-theory.

An interactive you play with one thumb, on Wundr.