Here's a home made catapult that my husband made for a science experiment for our son. A cup, a small pvc pipe and part of our extra blinds.
It was interesting to watch Zachary. As a four year old he had to learn which side of the catapult had to be up in order to fling the object up in the air. He thought the cup had to be up at first and then thought he had to hit the side with the cup on it. He got it after trying a few times. We really enjoyed seeing things fly across the room!