Put some food coloring into water, and freeze the water in an ice cube tray. After the water is frozen, place one of the colored ice cubes in an aquarium filled with water. Draw a picture of the cold currents indicated by the flow of the color from the melting cube. From your observations, you can infer that cold water is heavier than warmer water.

Put a one-hole stopper in a small bottle filled with hot water dyed with food coloring. Submerge the bottle in an aquarium of cold water. What happens?

The temperature of the ocean varies by as much as 65 degrees
Fahrenheit (18 degrees Celsius) between the equator and the polar regions. You might infer that, because it is heavier, the cold polar water constantly pushes toward the ocean floor and tends to move toward the equator, while the warmer, lighter surface waters at the equator tend to stay at the surface and flow toward the poles.

It used to be thought that the water, south of the equator, was literally boiling, and was infested by serpents. The old mariners would never sail south of the equator because of their fears. Prince Henry, the Portuguese guy who was the grandfather of navigation, dared to sail below the equator, and found that there was no boiling water and the "serpents" were basking sharks and other gigantic fish that liked warmer waters. That was an extra little bonus of information for
you.

Students need to research further to find out what the ocean
temperatures are at the equator; is the Pacific warmer than the Atlantic? While you are looking for that information, find out the ocean temperatures at the poles.