Lava lamps

My roommate has a lava lamp and I became mindful of how they work.  Lava lamps have colorful wax in them; this wax floats around in a clear liquid.  The wax moves up and down due to the heat in the lamp.  When the wax heats up, it becomes less dense than water because wax expands when it is heated. The wax becomes more fluid and floats to the top of the lamp, where it cools down and then descends.