Because the way they glow and how if you buy a red one it looks like lava. I say that the part that moves is the lava, and the liquid when lit with the light bulb turns into a lamp. as you can see in...
Colored paraffin wax and waterThe lava in a lava lamp is made out of oil, water, and carbon tetrachloride, commonly used in fire extinguishers.
Oil and wax.
You can't purchase a lava lamp refill, you would need to buy a new lava lamp.
Water, wax, and carbon tetrachloride.