Steve Howell, age 9, of Castleberry, Ala., for his question:

WHAT MAKES A THERMOMETER GO UP AND DOWN?

On the side of the glass tube there is a ladder of numbered degrees. As the thin thread of mercury rises and falls inside the glass tube, it shows just how many degrees warmer or cooler it gets. This happens because mercury changes as the temperature changes. When things get warmer, they tend to expand and get a little bigger. As they grow cool, they tend to shrink and get a little smaller

At ordinary temperatures, silvery mercury is a liquid metal. It, too, tends to swell a little as things get warmer and shrink as they get cooler. This makes it just the thing to work inside a thermometer's glass tube. As things get warmer, the mercury swells up and rises inside the glass tube. As things get cooler, it shrinks and the mercury falls.