Why you and I exist. According to "The Hitch Hikers Guide to the Galaxy," the mice build Deep Thought, a gigantic mega computer designed to divulge the answer to life. After 7.5 million years of calculation, it states the answer: 42. Here is the actual quote, Loonquawl being a mouse:

'"Forty-two!" yelled Loonquawl. "Is that all you've got to show for seven and a half million years' work?"
"I checked it very thoroughly," said the computer, "and that quite definitely is the answer. I think the problem, to be quite honest with you, is that you've never actually known what the question is."'