Did you know that the lava lamp made its first TV appearance on the popular science fiction show Doctor Who? (Hmm, does it look bigger on the inside to you?) It was invented in 1963 by a British accountant named Edward Craven Walker. He first called it the Astro. Although the name has changed, the key ingredients that makes lava lamps so mesmerizing remain the same.
Lava lamps are a trademarked model of motion lamps. Also called liquid motion lamps, these glowing gadgets usually contain two types of liquids. Nope, you don't have to make a trip to a volcano! The whirling globs are mainly made of paraffin wax. The wax floats in a water-based solution. The two substances have to be insoluble in one another, meaning they don't mix, like oil and water! But oil and water have different densities. (Density is a measure of how much matter is in a certain space.) In motion lamps, chemicals are added so that the two substances are similar in density while staying insoluble.
So what causes the wax to float and fall? At room temperature, the globs of paraffin wax are denser than the surrounding liquid. When the lamp is turned on, the light bulb generates heat. The bottom of the mixture gets hot and the wax begins to expand. As the wax expands, it becomes less dense, and it begins to float up.
Motion lamps are designed so that the temperature at the top is a bit cooler than at the bottom. As the wax is floating, it cools down and becomes more solid. When the glob reaches the top of the lamp, it is denser than the surrounding liquid and so it begins to sink back down to the bottom. When it reaches the bottom of the lamp, the cycle repeats! Science at work, no sonic screwdriver needed.
You can make your own version of a liquid motion lamp using household ingredients. Check out some books with cool science experiments and be sure to ask a grown-up for help!