Has been posted before, but one of my favorites.

Sherlock Holmes and John Watson go on a camping trip. In the middle of the night, Sherlock wakes up John and says, "Watson, what do you see when you look up at the sky?"
Watson looks up and says, "I see millions of stars."
Sherlock says, "Well, what can you deduce from that?"
After a moment's thought, Watson says, "Since there are so many stars,logically some of them must have planets orbiting them. And if some of those stars have planets, logically some must be able to support life. And if some could support life, logically some must. Therefore, I deduce that we are not alone in the universe. What do you think?"
Sherlock says, "No, Watson, you idiot, it means somebody stole our tent."

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