Place a drop of blood on a clean microscope slide and cover with slip. Human blood is GREAT for this, but sometimes a little hard to come by. Animal blood works just fine. You can get some from your friendly neighborhood butcher or meat shop if you don't want to spend money at a science supply house.
Now, place the slide on a microscope and focus under high power so the blood cells are visible. While observing the cells, have a partner place a drop of DISTILLED water along the edge of the cover slip. Observe the cells burst like little balloons as a result of osmosis. This is very dramatic when used with a flex cam and monitor atached to the microscope. These reasonably priced and are available thru Ward's Science Supply. If interested, contact Cheryl Smith, the best darn science supply person around!