It was nearly forgotten.

Archimedes Displacement Principle

A question posed to a software engineer position at IBM - I was stumped. I asked my brother and he gave me this clue which I immediately went to google it out. Eureka's can - I learnt it when I was 13 but having no reference to it for the past 10 years, the term felt relatively far away. Good to have it all back - hope it did to you too.