Add red food coloring to the bottle with very warm water.
Put an index card on top of the bottle filled with cold water.
While holding the index card in place, slowly and carefully turn the bottle upside down and sit it on top of the very warm water bottle. The index card should now be in between the two bottles.
Slowly remove the index card from in between the two bottles. Watch in amazement!
The reason why the warm water rises and cold water sinks is that the warm water is less dense than cold water. Density is something that makes things float. So, that means that warm water actually floats in cold water.
So, we think the temperature of the water makes the water rise and sink, not the red food coloring that we added to the warm water. Here's how you can test this explanation: Try this using cold clear water and cold red water, instead of warm red water. What do you think will happen?