A lava lamp contains a waxy substance and water which do not mix and a light bulb at the base as the bulb heats the waxy substance it rises near the top of the lamp the waxy substance cools and falls explain why this happens

This happens because of the density variation as the waxy substance is heated in the lava lamp.

Density is one of the intensive property of matter that often times varies with temperature changes.

One of this clear cut examples is seen in water and ice. Ice is less dense than water but they are made of the same chemical substances.

Density is the mass per unit volume of a substance. When the wax in the water is heated, the density reduces. A substance whose density is less than another will float on it by firstly rising if it is at the bottom. As it reaches the surface, away from the heat source, it cools and falls back into base where it is heated and rises again.