Sherlock Holmes and Dr. Watson are going camping. They pitch their tent under the stars and go to sleep. Sometime in the middle of the night Holmes wakes Watson up.
Holmes: "Watson, look up at the stars, and tell me what you deduce."
Watson: "I see millions of stars and even if a few of those have planets, its quite likely there are some planets like Earth, and if there are a few planets like Earth out there, there might also be life. What does it tell you, Holmes?"
Holmes: "Watson, you idiot, someone has stolen our tent!"
Voted 2nd best joke, from Internet Humor Study, by the British Association for the Advancement of Science, with Mr. Richard Wiseman (2002).