Science: how does a lava lamp work?

Self-tutoring about science in everyday life: the tutor mentions lava lamps.

One might say I inherited a lava lamp recently. I’ve never had one. When asked if I wanted it, I asked to see it demonstrated. Its owner turned it on and, after a while, portions of the blob at the bottom (purplish coloured, in this case) started rising up and so on. I thought it was neat, so I accepted it.

Lava lamps work by convection. The heat source, as I understand, is a light bulb stationed underneath the column of liquid.

Simply conceived, the clear, suspending fluid is water, while the coloured blob is wax. When the lamp is cool, the wax is solid and denser than water.

When the lamp is turned on, the light bulb starts to heat the column from below. Eventually the wax, sitting at the bottom of the column, heats up enough that it starts to melt.

When molten, the wax is less dense than water; therefore, the molten portions of it rise from the main blob up to the surface of the water. However, up at the top, the water is cooler, being further from the light bulb. Therefore, the molten wax cools, contracts, and becomes once again denser than the water. Then, the wax sinks back down to the bottom of the column.

Source:

kids.tpl.ca