I ran across this while looking for physics demos. I can't use it, but it seems to have so many interesting possibilities - like limiting interactions to 6, etc (the 6 degrees of Kevin Bacon, thing). TRANSMISSION OF INFECTIONS This activity demonstrates the transmission of infectious agents and is great to use with the entire class. It can take as long as you want it to to run. Use it as a minilab or an introduction to a new unit. Materials needed include clear plastic cups or beakers, saltwater, fresh water and a small amount of silver nitrate solution. In advance, speak to two students. Ask one not to interact (mix) with anyone and ask the other to interact with as many people as possible. The one student whom you have asked to interact will begin with a half a glass of clear saltwater while the rest of the of the students will receive the same amount of fresh water in their cups. Ask the students to interact with other students by pouring their water into the other persons glass and then having the other person pour half the liquid back into their glass. After a few minutes stop the interactions. Drop one drop of the silver nitrate solution into each cup and explain that if it turns cloudy then they had become infected. Many interesting avenues can be taken with this activity. If students keep track of whom they interacted with and in which order, the class can actually determine who was the original infected person. This is similar to how health officials try and trace the progress of an infectious disease.