Which way did the bicycle go?
A bicycle rides across a patch of wet sand, leaving two overlapping tire tracks — one from the front wheel, one from the rear. The two curves cross each other several times, and neither track is labeled front or back. Just from the shape of the crossings, can you work out which direction the bicycle was traveling?
Reveal the answer
The rear wheel is rigidly linked to the front by the bike's frame, so it's always being 'towed' toward wherever the front wheel currently points — which forces the rear track to curve more sharply (a smaller radius) than the front track at every corresponding point. Once you spot which of the two tracks is the tighter, more sharply bent one, that identifies the rear wheel's path, and the way it bends at each crossing points you toward the direction of travel. This tire-track puzzle is one of the best-known problems in Boris Kordemsky's 1956 collection The Moscow Puzzles.
— Boris A. Kordemsky, The Moscow Puzzles — First published in Russian, 1956; English edition 1972
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