Place tray on top of 3 evenly spaced, inverted styrofoam cups. Fill the 4 th cup with hot water and slide under the center of the tray. Using the eyedropper, gently place a blob of food color inside the tray, on the bottom directly over the heat source. Variations on this experiment include moving the heat source, using aquariums or ice. The central heat source causes the warmed water to rise in the center.
Warm water is lighter, less dense than cold water.
The warmed water molecules are excited and move faster. After they rise, they move along the surface to the edges where they cool, become denser and sink to the bottom. Being on the bottom they become part of the source being heated and are drawn towards the center. This can be sketched in a doughnut type pattern.