LifeWiki:OCAFA
Brian's Brain (also called BB, or simply Brain) is a cellular automaton and one of the best-known Generations rules. It is similar to Seeds, but with an additional state; dead cells get born if they have exactly two live neighbors, and live cells never survive, but instead of dying immediately, they advance to a third state, not considered "live" for the purpose of cell birth, before dying. The rule was first considered by Brian Silverman in the mid-1990s.
Brian's Brain can be thought of as emulating a neural network, with state 0 representing "ready", state 1 representing "firing", and state 2 representing "refractory"; the rule can then be stated as "only a cell in the ready state may fire and it will only do so if exactly 2 of its neighbors are firing. After firing for one step, a cell spends a step in the refractory state before regaining readiness".
Usually, a random starting configuration in Brian's Brain will explode at the speed of light (like the image), emitting many spaceships, rakes, breeders, puffers and wavestretchers. Additionally, there is a common c/4 diagonal spaceship called the butterfly.
Although the rule is relatively old, it has never been systematically explored. Due to all patterns being phoenixes, it is not very easy to construct stable patterns in it.