"I felt exactly like that other brilliant fellow must have felt when he discovered penicillin. Eureka! I cried"
Archimedes in his Bath
Public DomainArchimedes in his Bath - Credit: John Leech

Penicillin, the first antibiotic to be used in medicine, was discovered in 1928 by Alexander Fleming. But he wasn't famous for shouting "Eureka" (Ancient Greek for "I have found it").

That comes from a story about the Ancient Greek mathematician Archimedes, who is said to have leapt from his bath and run around town naked shouting "Eureka" when the bath helped him discover how to measure the volume of an irregular object: the solution is to place it in a full bath and measure how much water it displaces.