We have been learning about simple machines as part of our science.  Harry wrote this explanation showing how a catapault works.
A catapault is a device that fires objects up into the air. 

To make one at school you need these objects:- A ruler, a counter and a glue-stick.  First you put the glue-stick down.  Then you put the ruler on top of the glue stick.  Place the counter on top of the ruler on the outside of the ruler on the bit where it is hanging down.  Then you push the side that's hanging up.  The counter will fly.

The simple machine in a catapault is a lever.