Inspired by
Daryn's post in
this awesome thread:
This is a popular question in freshman physics. Imagine this set-up (because I don't want to take the time to MS Paint it): You have a wall at the front end of a cart and you're standing on the back end of this cart facing the wall and you have a bunch of perfectly elastic tennis balls. You throw the balls against the wall and they bounce off. Describe the motion of the cart, assuming perfect bearings and a smooth, no-slip condition between the flat ground and the wheels.
If you can't figure out what I'm talking about, imagine Daryn's fan on skateboard picture and replace the fan with a little person throwing tennis balls against the "sail".