Building a catapult

I have to build a catapult for my physics class. It has to be able to shoot a ping-pong ball. It's not that I don't like this project, this is the most awesome project I ever had before. However, I have to find a design. I probably looked at about 20 designs already. At least I do not have to worry about diffuculty because my dad is an engineering wiz and can help me build it. Then I have to test it. That is the most awesome part by far because I get to shoot it. I see why the Mythbusters have so much fun now with testing things.