Naomi James, age 13, of Bowling Green, Ohio, for her question:

WHAT IS VISCOSITY?

Viscosity is a property of fluids that causes them to resist flowing. It is caused by internal friction from the fluid's molecules moving against each other. All fluids, including liquids and gases, have some degree of viscosity.

Fluids with high viscosity, such as molasses, flow more slowly than those with low viscosity, such as water.

The degree of viscosity is important in many applications. The viscosity of motor oil, as an example, determines how effectively it can lubricate the parts of a car's engine.

The more strongly a fluid's molecules interact, the more viscosity of the fluid.

The temperature of a fluid also determines how strongly its molecules interact. Hot liquids have lower viscosity than cold liquids. However, gas molecules interact more strongly at high temperatures. Hot gases have higher viscosity than cold gases.