Gel beads have been described above. They are beads that contain a gel that absorbs heat to change from a solid to a liquid state* within the gel capsule. This cools you as you sleep. The gel inside the bead, or capsule, then reverts back once you are up and the mattress reverts to its normal state. The material making the gel is known as a phase-change material. When the material reaches the temperature at which it changes phase from solid to liquid, it absorbs a lot of heat without actually changing temperature. This is just like solid hail or snow absorbs a lot of heat to melt it, but the temperature of the water involved remains around the 0C level. When you get out of bed, the temperature around the gel foam drops, and it reverts back to the solid-state. The gels involved liquefy and solidify at higher temperatures than water does.