This week's experiment is one that you can try the next time you take a bath.

To try this, you will need:

- some warm water
- a washcloth or paper towel

Have you ever noticed that you feel cold after a hot bath? Actually, there is more than one reason that you could feel chilled when you get out of the tub. If the water was very warm, then the air will feel cooler by comparison, but even if the water was cool and the air was warm, you could still feel a chill. To see why, dip the washcloth into some warm water. Squeeze out the excess and place the washcloth on your face. It should feel nice and warm.

To cool the cloth, hold it by one edge and swing it back and forth a few times. Then place it back on your face. You should notice that it is now quite cool.

Why would waving the cloth in the air make it cooler? This will work even if the air in the room is quite hot, so the air itself is not cooling the cloth. Instead, it is being cooled by evaporation. When liquids evaporate, they soak up quite a bit of heat. As you swing the wet cloth through the air, some of the water evaporates. This removes quite a bit of heat from the cloth, leaving it nice and cool.

This is the idea behind sweating. When you get hot, you sweat. The sweat evaporates and cools your skin. It is also the principle used in the mist cooling systems that are becoming popular. They spray a very fine mist of water into the air. The water evaporates and cools the air, making you more comfortable on a hot day.

One way to cut down on the post-bath chill is to keep the bathroom nice and steamy until you are dry. If the air is already very humid then the water will not evaporate from your skin and you stay warmer.