Put two equal sized containers that have flat, wide rims in a large bowl. Fill one container with hot water and the other with cold. Put a drop of red food coloring into the hot water and blue food coloring into the cold water. Watch the speed in which the food coloring mixes into the water. The red coloring mixes quickly because the hot water molecules are moving rapidly, causing a current in the water. The cold water molecules are still so there is no current and the food coloring does not mix well unless it is stirred.
Use the hot and cold water in the containers from the first experiment. The hot water may have cooled slightly, so empty some out from the container and top up with more hot water; add a little more red food coloring if necessary. Place a sheet of cardboard over the cold water container. The card needs to completely cover the top of the container. Lift up the container, making sure the cardboard stays firmly in place, then carefully turn the container upside down and place it on top of the hot water container. Gently pull out the cardboard from between the two containers. The blue water races to the bottom container and the red water races to the top container, but the water becomes a purple color as the two currents mix.
Empty the water from the containers you used in the last experiment and refill one with cold water and the other hot water. Put blue food coloring in the cold water and red food coloring in the hot water as before. Put the cardboard onto the top of the hot water container. Carefully lift up the container, turn it upside down and place it on top of the cold water container. Gently pull out the cardboard from between the containers. This time the blue and red colored water remain reasonably separate and distinguishable. This is because hot water is less dense; it weighs less than cold water, so it stays in the top container and therefore there is not a flow of current.
Fill a large glass bowl about 3/4 full with tap water. A large mixing bowl or goldfish bowl is fine. Put a coffee mug or cup upside down in the jar. Get two small, clean empty glass or plastic bottles, similar in size to the ones containing the food coloring. Put a drop of red food coloring into one of the bottles and a drop of blue food coloring into the other. Fill the bottle that has red food coloring with hot water and other with cold water from the fridge, as it needs to be colder than the water in the glass bowl. Put your fingers over the top of the bottles and then lower them gently into the glass bowl. Lay the bottles down side-by-side on top of the mug or cup and then remove your fingers from the tops of the bottles. The colored water flows from the two bottles, but they flow in different directions. The red water rises to the surface; the blue water flows to the bottom.