Weigh

nine coins, one heavy. two weighings. go.

find the fake with a balance scale — log₃, felt

A balance only ever gives one of three answers — left, right, or level — so each weighing narrows nine suspects down by a factor of three rather than two. Two weighings, chosen well, are exactly enough to land on one coin out of nine; a third would be more than the arithmetic needs.

Concepts: logic, information-theory.

An interactive you play with one thumb, on Wundr.