Riddles, Puzzles, Brain Teasers, Oh My!

I open each psychology class with a riddle/puzzle/brain teaser/etc. I am finding it difficult, however, to find good ones! I thought it would be very easy, but it turns out that is not the case. Therefore, I need your help! Do you have a favorite brain teaser? A puzzle that always makes people think? A riddle with a clever answer? Let's hear'em in comments.