Twelve baskets circle a pond, one fish each — redistribute them in two fish per basket
Twelve baskets are arranged around a fish-pond, each holding exactly one fish. Moving around the circle in one direction, you must pick up a fish, carry it over exactly two baskets, and drop it into the next one — repeating this until only six baskets remain in use, each with two fish, and you end up back where you started. What's the fewest laps around the pond this takes?
Reveal the answer
Three laps. Numbering the baskets 1 to 12, moving fish four places at a time around the ring sorts them into six full baskets in three full circuits — a four-lap solution is easy to find, but three is the minimum. From Henry Dudeney's The Canterbury Puzzles (1907).
— Henry Dudeney, The Canterbury Puzzles — 1907, 'The Riddle of the Fish-pond'