David Spears, age 9, of Centerville, Tenn., for his question:

Why does mercury move up and down in a thermometer?

In our warm and cool everyday world, the only liquid metal is silvery mercury. The shiny stuff is dangerous to touch, so it is best for young people to study it from behind glass walls. A thin thread of liquid mercury is shut safely inside the glass tube of a thermometer.

Mercury swells a little as it gets warmer and shrinks a little as it gets cooler. When you have a fever, your body is warmer than it should be. Someone puts a thermometer into your mouth, and your extra warmth warms the mercury. It swells and climbs farther up the glass tube. If you dunk the thermometer in ice water, the mercury will shrink with the cold. It will take up less space, and the fine thread will drop lower in the glass tube.