Eureka effect
Archimedes was asked by the local king to detect whether a crown was pure gold, or if the goldsmith had added silver. During his trip to the public bath, he noticed how water got displaced when his body sank into the bath. The volume of water displaced equals the volume of the body immersed in the water. He suddenly realized that two objects which weighed the same in air might not weigh the same in water, because the less dense object would displace less water. This is known as Archimedes' principle.