“I have found it! or I have got it!, commonly quoted as Eureka!”
Archimedes (-287–-212 BC) Greek mathematician, physicist, engineer, inventor, and astronomer
What he exclaimed as he ran naked from his bath, realizing that by measuring the displacement of water an object produced, compared to its weight, he could measure its density (and thus determine the proportion of gold that was used in making a king's crown); as quoted by Vitruvius Pollio in De Architectura, ix.215;