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Water Balloon Catapult

Simple machines make work easier. There are six kinds of simple machines: lever, pulley, screw, wedge, inclined plane, and wheel and axle. You can combine simple machines into something more complex called a compound machine; for example, a bike is made up of screws, levers, a pulley and a wheel and axle. Build a simple machine to launch a water balloon!


  • Two identical gallon or half-gallon milk jugs, with handles
  • Yardstick
  • 20-inch dowel rod
  • Two large rubber bands
  • Paper bowl or cup
  • Duct tape
  • Water balloons
  • Water


  1. Fill the jugs with water, cap them and put them on the ground or a table.
  2. Attach the yardstick perpendicular to the dowel using two rubber bands that make an X shape. Set the dowel at the 8-inch mark. Rest each end of the dowel inside the jug handles.
  3. Tape a paper bowl to the other end of the yardstick. 
  4. Hold the bowl end down and load it with a small water balloon.
  5. To launch, just push down on the other end of the yardstick and get out of the way!

This can be hard to visualize! Check this document for some additional visual instructions.

What's happening?

A catapult is a type of simple machine called a lever. A lever is a bar centered on a turning point called a fulcrum that's used to raise or move weights. Levers make it easier to lift heavy things, like a person or a seesaw. In the catapult, the bar (yardstick) pivots on the fulcrum (dowel) in order to raise the bowl and toss the water balloon.


This is best done outside. Don't aim the catapult at anyone!


Make a target and see if your aim can hit the bullseye!

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