You will need:
1. Cut the bottom off the plastic bottle and cut the top off one of the balloons.
2. Stretch the balloon tightly over the bottom of the container so it forms a thin skin like the surface of a drum, and secure very firmly with stickytape.
This balloon represents the diaphragm.
3. Make a hole through the bottle lid and attach a straw. Place the other balloon inside the lid, with the top of the balloon attached to the straw with the rubber band - the balloon should sit inside the bottle. This balloon represents the 'lung'.
4. Carefully pull the balloon at the base of the bottle and watch the 'lung' balloon inflate.