light & heavy water
Find two bottles whose mouths are the same size.
Put hot water into one bottle. Mix water soluble paint,
ink or food coloring with the hot water to make it more
visible. Put cold water in the other bottle. Both bottles
should be completely full. Put a thin card or piece of thin
stiff plastic across the top of the bottle of cold water.
Hold the card in place while you invert the bottle and place
it on top of the other bottle. Carefully pull out the card.
The denser cold water slowly sinks while the hot water rises
to float on top of the cold water. As the colored water rises
it forms interesting shapes something like rising smoke.