A tiny density shift drives the whole cycle
A lava lamp runs on a delicate balance: its wax and surrounding liquid are formulated to have almost the same density. Density is how much mass fits into a given volume. When the lamp is cool, the wax is dense enough to rest at the bottom. The bulb heats it, and the wax expands. Its mass stays essentially the same while it takes up more space, so its density falls—and it becomes buoyant enough to rise.
That near-match matters. If the wax were much denser than the liquid, heating might not make it buoyant; if it were much less dense, it could remain at the top. The small difference lets modest temperature changes flip which fluid is denser.
The lamp’s shape completes the loop
As a blob rises, it moves away from the bulb and into cooler liquid near the top. It loses heat, contracts, and becomes denser. Once it is heavier for its size than the surrounding liquid, it sinks. At the bottom, the bulb warms it again, starting another trip.
The wax and liquid do not dissolve into each other, so the wax stays in visible blobs rather than blending into the fluid. Their changing density supplies the motion; the lamp’s heat source keeps the cycle going. It takes time to warm up, and while it runs, the lamp continually shifts heat from the bulb through the liquid toward the glass. The blobs rise and fall because the lamp keeps nudging two nearly balanced fluids across the point where one can float in the other.
