I was pleased to come across this nice list of The Best Thought Experiments on Wired recently. Many of which are cool to think about and don't require a hefty science background. But for those who are jonesing for some real deep cosmic thinking look no further than Wikipedia (especially those philosophy ones, yikes!). Thankfully for us practical types who don't like to think too hard, I remembered a great thought experiment from  Surely You're Joking Mr. Feynman which is especially appropriate for these steamy summer days:

You have an S-shaped lawn sprinkler-an S-shaped pipe on a pivot-and the water squirts out at right angles to the axis and makes it spin in a certain direction. Everybody knows which way it goes around; it backs away from the outgoing water. Now the question is this: If you had a lake, or swimming pool-a big supply of water-and you put the sprinkler completely under water, and sucked water in, instead of squirting it out, which way would it turn? Would it turn the same way as it does when you squirt water out into the air, or would it turn the other way?

To be honest, I haven't quite figured it out! And Feynman only uses the example to lead into some hilarious lab hi jinx with exploding water bottles. Help me out!