The lungs are just one part of the respiratory system. The sheet of muscle located below the lungs is called the diaphragm and actually does most of the work of breathing. Your lungs are in a cavity and expand when the muscles of the diaphragm move down. They contract when the diaphragm moves up and pushes the air out. Coffee-can and soda-bottle lung models are easy to make and can show you how the diaphragm and lungs work together when you breathe.
Cut both ends off the coffee can to make a large cylinder.
Place the plastic bag inside the can and fold the top over the edges of the can. Hold in place with two rubber bands. The bag should be fairly loose inside the can.
Cut off the mouth of the balloon, and stretch the broken balloon over the opposite side of the can from the plastic bag. Secure the broken balloon in place with the other two rubber bands.
Grab the balloon and pull down. The plastic bag should inflate to simulate taking a breath into the lungs. Gently push the balloon up into the can. The bag should deflate and simulate exhaling.
Carefully cut the soda bottle in half with scissors. Dispose of the bottom half.
Place the balloon on the end of the straw and secure with a rubber band.
Poke the straw through a hole made in the clay, taking care not to plug the straw with the clay. You will need a clear passage for the air to travel through the straw. Make sure at least 8cm of straw is sticking through the clay. Place the straw through the mouth of the soda bottle so the straw is sticking out of the bottle and the balloon is hanging inside. Use the clay to secure the straw to the bottle and seal the edges.
Stretch the plastic bag across the opening of the bottom of the bottle half. A balloon with the mouth removed will also work. Secure the plastic or broken balloon with a rubber band. You can also reinforce the seal with the tape.
Pull or press on the stretched plastic bag or stretched broken balloon to cause the balloon inside to inflate or deflate to simulate inhaling and exhaling.
Be sure to dispose of the balloons appropriately, since it is a choking hazard for small children.