Inspired by the father of logic, the ancient Greek philosopher Aristotle, this intriguing little puzzle looks like a wooden honeycomb, with 19 hexagonal tiles numbered from 1 to 19. All you have to do is arrange the tiles so every row—whether 3, 4, or 5 tiles long—adds up to 38 (or twice 19). There are several solutions, though finding any of them will keep your synapses firing. 5½ inches square.