The king asks Archimedes to determine if a present he’s received is actually solid gold. Unsolved problem at the time. It tortures the great Greek mathematician for weeks – insomnia haunts him and he twists and turns in his bed for nights on end. Finally, his equally exhausted wife – she’s forced to share a bed with this genius – convinces him to take a bath to relax. While he’s entering the tub, Archimedes notices the bath water rise. Displacement, a way to determine volume, and that’s a way to determine density – weight over volume. And thus, Archimedes solves the problem. He screams “Eureka” and he is so overwhelmed he runs dripping naked through the streets to the king’s palace to report his discovery.

Source: Pi – IMDB