A monster on the road to Thebes killed everyone who answered this riddle wrong
In Greek myth, the Sphinx guarded the road into Thebes and put the same riddle to every traveller, devouring anyone who failed: what creature walks on four legs in the morning, two legs at noon, and three legs in the evening?
Reveal the answer
A human being — crawling on all fours as a baby, walking upright as an adult, then leaning on a cane in old age. Oedipus is said to have solved it, causing the Sphinx to destroy herself; the story is preserved in later Greek sources like Apollodorus's Bibliotheca, though the riddle itself was passed down orally long before it was ever written.
— Pseudo-Apollodorus, Bibliotheca — Greek mythographic compilation, c. 1st-2nd century AD