A simple demonstration of the fact that the work required to increase the size of a balloon (or bubble) is proportional to 1/R (R is the radius of the balloon). Beacuse of this (in theory) it ought to be impossible for a liquid to boil.

Two new balloons (same size)
Large stopcock
Wire or twist-ties to hold inflated balloons to stopcock arms

This demonstration should accompany a discussion of the boiling phenomenon. It illustrates why bubbles usually form on some sort of seed point, dust particle, or whatever in the liquid.

Inflate the two balloons to different sizes. One should be much smaller than the other. Attach both to the arms of a stopcock (closed) and seal tightly with wire (see Figure). Ask the class what will happen when the stopcock is opened. Many will predict that the larger balloon will contract by forcing air into the smaller until both are the same size. Surprisingly, the reverse actually happens. The smaller balloon forces its air into the larger so that the small balloon gets smaller and the large balloon gets larger.

The point to make is that the smaller the diameter of the bubble, the harder it is for it to expand. Taken to the logical end, this means that bubble formation in a liquid ought to be impossible and that liquids should never boil. In boiling liquids and in water solutions of carbon dioxide (like Coke or beer), bubble formation usually happens at some imperfection or scratch on the inside of the glass vessel. Boiling a liquid in a brand new, never used flask can often be very rough and uneven. Another connection to make is to explain why one vigorously stirs or agitates water before heating it in a microwave oven. Without a few tiny seed bubbles, the water often "bumps" when boiling commences usually resulting in the inside of the microwave oven being doused with hot water.

I can’t imagine any hazards in this one. Maybe if you blow too hard into the balloon, you might develop a hernia. The balloons can be reused if they are not too old.