Hello! Here's a quick video of a breathing model I made to show that when the diaphragm contracts, it changes the air pressure in the thoracic cavity, and air rushes into the lungs.
Here is one tutorial -- there are many on the internets:
I definitely want to upgrade mine now that I know I can make a pretty one like this:
To my students: you can check out the Youtube playlist on the Current Students tab, listed under Resources.