Water balloon heat capacity
To demonstrate that the water will absorb the heat so the rubber does not have to absorb the heat.
A lighter or matches
First fill one balloon with air and fill the other with water. Then bring the flame close to the balloon filled with air and the balloon should pop after a few seconds. Then repeat the same process with the water balloon but this time it should not pop.
A nice entertainment part ot this lab is after the air balloon pops ask for a volunteer to hold the water balloon since most of the students will think that it will also pop.
The air became to hot to quickly and therefore the rubber got hot also and the balloon popped. The water balloon did not pop because the water absorbed the heat of the flame, and since water takes a great deal of heat to raise it's temperature a small amount it did not make the balloon pop. This also explains why a water bed is always cool because the water is harder to raise in temperature then air or other materials.