I cut the bottom off the bottle, and removed the cap, but left the balloon where it was. Then I covered the bottom of the bottle, a little loosely, with plastic wrap, securing it in place with a rubber band. Finally, I taped the top part of a bendy straw onto the middle of the plastic wrap. And voila, we had a model of a lung, rib cage, and thoracic diaphragm.
The balloon represents the lung. The pop bottle represents the rib cage. The plastic wrap represents the thoracic diaphragm, the thin sheet of muscle extending across the bottom of the rib cage.
When you pull up, gently on the straw, it demonstrates what happens when you breath in. The diaphragm contracts, making more space in the chest cavity, and the lung fills with air.
When you drop the straw back down, it's like breathing out. The diaphragm relaxes, reducing the space in the chest cavity, and air is forced out of the lung.
Of course now, even after the children have tired of inflating and deflating the lung, I can't bring myself to throw the thing away. After all, it might still have an experiment, or two, lurking in its frame. I guess I'll just pop it back under the sink to be discovered the next time I try to straighten up a little.
Oh, and by the way, I did not think up this little experiment on my own. It's one that came from some science kit or other, long enough ago that the kids can't remember ever having done it, and I can't remember who to give the credit too. But all the same, someone out there thought of it first, and whoever it was, has my thanks.
It's great to be a homeschooler.