I place water in a fish tank and place two styrofoam cups in the fish tank with 50 gram masses to hold the cups on the bottom.  The cups in the fish tank should be above the water level in the fish tank.  Place hot water in one cup and three drops of red food coloring.  Place ice water n the second cup and add 3 drops of blue food coloring.  Make a thumb-tack puncture in the middle of the cup.  The cold blue water rises and the hot red water sinks.