"The eureka effect (also known as the aha! moment or eureka moment) refers to the common human experience of suddenly understanding a previously incomprehensible problem or concept. The Eureka effect is named after the myth that the Archimedes, having discovered how to measure the volume of an irregular object, leaped out of a public bath, and ran home naked shouting "eureka" (I found it)." Wikipedia
This page has a collection of videos which demonstrate the Aha Moment.
Disney Pixar - For the Birds