In optical filters and theatrical lighting a color gel is a transparent or translucent colored panel used to change the color of transmitted light. A gel (from the Latin gelu "freezing, cold, ice" or gelatus "frozen, immobile") is a colloidal system in which a porous network of interconnected nanoparticles spans the volume of a liquid medium. In general, gels are apparently solid, jelly-like materials. Both by weight and volume, gels are mostly liquid in composition and thus exhibit densities similar to liquids, however have the structural coherence of a solid. An example of a common gel is edible gelatin. Many gels display thixotropy - they become fluid when agitated, but resolidify when resting. By replacing the liquid with gas it is possible to prepare aerogels, materials with exceptional properties including very low density, high specific surface areas, and excellent t