Sorry to disappoint you, but there is no lava in a lava lamp. The gooey substance that floats around inside a lava lamp is actually a type of coloured wax. The rest of the lamp contains other liquids such as water. The light from the lamp heats up the mixture of the wax and the surrounding liquid, causing a change in density. This makes the wax float upwards towards the top of the lava lamp. As it reaches the top, the wax cools down and drops down. As it reaches the light again, it warms up and float upwards. This is why the contents of a lava lamp are always moving around. The shape of a lava lamp is designed such that the wax will always repeatedly move upwards and drop downwards as long as the lamp is switched on. Lava lamps can be costly to make because a lot of engineering goes into their construction, from the type of wax used to the shape of the lamp to the amount of heat the light source needs to provide, to manufacturing the lamp such that liquid in the lamp does not come into contact with the electronics.