What you need:
- Paper cup
- Bending straw
- 2 toothpicks
- Rubber band
- 4 cardboard circles
- Take a paper cup and insert 2 toothpicks as shown in the image.
- Insert the 4 cardboard circles into the toothpicks.
- Insert a bending straw within the cup and stick it from inside with tape.
- Tie a balloon at the end of the straw with a rubber band.
- Blow air from the open end of the straw (action) and see the cart move (reaction).