The Game of Life
Tap or drag on the grid to switch cells on and off. Press Step to move time forward one tick, or Run to let it go. Or load a ready-made pattern: a blinker, a glider, or a gun that fires gliders. Before you press Step, try to predict the next picture. Nobody steers anything: every cell just counts its live neighbours.
What's going on
Every cell has eight neighbours: the cells that touch it, corners included. At each tick, every cell counts how many of its neighbours are alive, and all cells change at the same moment.
The four rules. A live cell with fewer than 2 live neighbours dies (too lonely). A live cell with 2 or 3 live neighbours lives on. A live cell with more than 3 live neighbours dies (too crowded). A dead cell with exactly 3 live neighbours comes alive.
That is the whole game. John Conway invented it in 1970, and people are still finding new patterns in it.
Block. Each of its four cells has 3 live neighbours, and no dead cell nearby has exactly 3. Nothing changes. A pattern like this is called a still life.
Blinker. Three cells in a row flip between flat and upright, forever. It repeats every 2 ticks.
Glider. Five cells that rebuild themselves one square down and one square across every 4 ticks. Nothing in the rules says "move", yet it walks across the grid.
Glider gun. Conway offered a prize to anyone who could show that a pattern can grow forever. Bill Gosper's team won it in 1970 with this gun, which fires a new glider every 30 ticks.
Normally anything that walks off the edge of the grid is gone. Tick edges wrap around and the left edge joins the right and the top joins the bottom, so a glider comes back round. Try the gun with wrapping on and see what its own gliders do to it.
Sheet: A universe with four rules.