Cellular automata are little programs that follow a set of rules for cells to interact with each other. It can be seen as a sort of zero-player "game". The most famous example is John Conway's Game of Life, where there is a square grid where if a "live" cell has two or three neighboring "live" cells, it stays alive, and if it has any other number, it "dies". If a "dead" cell has three neighbors, it becomes "alive" again.