School Science/Bell jar model lung
This simple Bell jar model lung can be used to demonstrate the basic principles of breathing. The model should be assembled as in the diagram. A swimming cap makes a suitable diaphragm. The cap can be attached to the bottom of the bell jar, and the balloons can be attached to the Y shaped tube, with elastic bands or plastic coated wire.
How the Model Works 
The cavity inside the bell jar is airtight
- As the diaphragm is pulled down, the volume of the cavity increases. This causes the pressure to fall. Air rushes in to equalise the pressure causing the balloons to inflate:).
- As the diaphragm is pushed up, the volume of the cavity decreases, the pressure rises, and the air rushes out of the balloons causing them to deflate.
- This is a great project for classes studying health or for those students learning fluids in science.
The bell jar model helps children from all ages
More Advanced Models 
To more accurately render the anatomical and physiological properties of the lung, where the intrapleural space is filled with aqueous fluid you might want to replace the air in the bell jar with water. By filling the intrapleural space with water the model becomes a little bit more responsive, because of the incompressibility of water in contrast to air. Additionally preloading the balloons by removing water also better assures their automatic return to the exhaled position. A good introduction and overview is given by an article by T. Sherman in Advances in Physiological Education. 
- Sherman, T. F. A Simple Analogue of Lung Mechanics. Am. J. Physiol. 265 (Adv. Phyaiol. Educ. 10): S32-S34, 1993. http://advan.physiology.org/cgi/reprint/265/6/S32?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&author1=Sherman%2C+T&fulltext=bell+jar+lung&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT