Fill a very small glass bottle half-full with water using an eyedropper. Add a drop of food coloring. Choose a bottle that has a tight-fitting lid, such as the type in which perfume samples come.
Fill a quart jar with cold water. Add the small bottle. If it doesn't float, remove a tiny amount of the water and keep trying until the small bottle floats. Test the temperature of the water and make note of it. When you are ready to teach your class, fill the bottle with water of this temperature. Explain to your young students that warm water is lighter and floats on top of the cold water.
Pour out some of the cold water and add warmer water. Have students experiment with trying to get the floater to bob in the center of the jar, or to sink to the bottom. The warmer the surrounding water, the further the floater will sink. This is how this type of thermometer, invented by Galileo, works.
Add 2 inches of water to the bottom of a small-mouthed quart jar. Add blue food coloring and stir to make the water bright blue.
Insert a clear drinking straw so that 2 inches are inside a glass soda bottle. Wrap the clay around it, holding it in place and sealing the jar.
Have students hold their hands around the jar with water in it to warm the glass. Let them take turns, taking a minute or two each. The glass should be as covered as possible by palms for at least a minute.
Turn the soda bottle upside down so the straw extends into the jar, and is in the water but not touching the bottom of the jar. As the water extends up the straw, talk to your students about how the water, warmed by their hands, is drawn up into the colder soda bottle. This is another type of thermometer designed by Galileo.