Photoncytosis is an idle desktop life simulation in which a tiny organism feeds on the light of the screen and grows while the player works. Each cell stores energy, and absorbed colors influence the organism's development. Players can inspect cells under a microscope, research mutations, move creatures toward light, and confine them to an application window. Specialized cells support harvesting, perception, movement, energy storage, and repair. Organisms can split, regroup, and compete for resources in a changing desktop ecosystem.