Puzzles

Roll one coin around another — how many times does it spin?

Hold one coin still on a table. Roll an identical coin around its rim, without slipping, until it returns to its starting position. How many full rotations does the moving coin make on the way around?

Reveal the answer

Two — not one, which is what almost everyone guesses at first. Travelling around a curved path adds one extra rotation on top of the spin from the coin's own circumference. Martin Gardner popularised the puzzle in his 'Penny Puzzles' column, collected in Mathematical Carnival (1975); it made national news in 1982 when a flawed SAT question offered no correct answer among its choices.

— Martin Gardner, Mathematical Carnival — 1975, 'Penny Puzzles'

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