Kevin Linden, age 7, of Peoria, Illinois, for his question:

Why do broken light bulbs pop?

The soft popping sound from a broken light bulb says "Danger!" Beware of dagger sharp thin, splintered glass. At least, that is what it would say if it were smart enough to . care. Actually, of course, the popping sound is not really a word, It is the noise made by a puff of air rushing to fill up an almost empty space. The pocket was the bulb before it was broken. It was tightly sealed from the air in the room.

Fire is fed by ordinary oxygen in the ordinary air. And fire gets so hot that it melts even metals. The light from a bulb comes from heating the fine filament of metal wire inside it. We want it hot enough to glow, but not hot enough to melt the metal. So the oxygen and air are taken out of the bulb. Some gases refuse to feed fire, so a trace of these nonburning gases is sealed inside the bulb. It is much thinner than the air outside. And all gases mingle together, trying to spread themselves evenly. When the bulb breaks, the air in the room puffs in with a pop to thicken up the thin mixture of gases that was trapped inside the glassy prison. The breaking glass may add a small crackle that makes the pop louder.