John Stewart, age 17, of Pittsfield, Mass., for his question:

CAN YOU EXPLAIN A LIQUID'S VISCOSITY?

Liquids are substances in a state of matter intermediate between the gaseous and solid states. The molecules of liquids are not as tightly packed as those of solids or as loosely arranged as those of gases. Liquids are characterized by a resistance of flow, called viscosity.

The viscosity of a liquid decreases with temperature and increases with pressure. Viscosity is also related to the complexity of the molecules constituting the fluid. The viscosity is low in liquefied inert gases and high in heavy oils.

The pressure of a vapor in equilibrium with its liquid form, called vapor pressure, depends only on the temperature and is also a characteristic property of each liquid. The boiling point, freezing point and heat of vaporization (roughly, the amount of heat required to transform a given quantity of a liquid into its vapor) of liquids are characteristic properties, as well.

Sometimes a liquid can be heated above its usual boiling point. Liquids in that state are referred to as superheated. Similarly, liquids can be cooled below their freezing point. These are called supercooled.