Well, Sir Isaac Newton was a terribly clever man, and invariably had, within his life, multiple ephiphanies. But I assume the one you're talking about is when he came up with the theory of gravity. As the story goes, he was sitting under an apple tree when an apple fell upon his head. He found himself curious, and wondered, "Why does this apple fall down? Why doesn't it fall up? Or sideways? Or just stay suspended mid-air?" This was technically the epiphany moment, and then he went on to hypothesize about gravity.
Hope this helps :)
