Cold water sinks because the water molecules are packed closer together (it's more dense). Warm water rises because the molecules are a bit farther apart and is less dense. In this video I placed an ice pack on one side of a glass vase (14 inches high, 10 inches wide and 1 inch deep) and let it sit for about a minute. I dropped blue food coloring on the cold side and red food coloring on the warmer side. The taping time was about 1.5 hours and time lapsed to about 30 seconds.