Paul Reitz, age 9, of New Hartford, New York, for his question:

What makes bubbles in boiling water?

Water is made of tiny water molecules, much too small for our eyes to see. When the water is a runny liquid, its molecules cling together and slither around playing follow the leader. As the water gets hotter, they use the heat energy to move faster. If the water reaches 212 degrees Fahrenheit, its molecules get up enough speed to separate. They change into an invisible gas called water vapor.

When water heats up to its boiling point, bubbles of gas form on the bottom of the pot, where things are hottest. Gases are much lighter than liquids or solids. So the bubbles in the boiling water float up to the top. They even are too light to float on the surface. So the separate molecules zoom off and mingle with the other invisible gases of the air. The boiling bubbles are pockets of water vapor separating from the liquid water.