I know the answer, but i wont give it away. Here is one: A professor, after giving a long speech, wanted to test one of his theories. His theory was that people after listening to something for a long time, weren't as good at mental excersises, as they were before. So, after he was done, he told people this riddle:
What five letter word, when you take away four of its letters, still is pronounced the same. For example, if it was two letters, and I said 'Oh', if you took away the 'h' it would still sound the same." After everyone had written down their answers, he asked them all to form a line to submit the answers.
P.S. I just made this up. It might be similar to others, but I don't know.