Greenberg-Hastings Model (Excitable Media)

This cellular automaton models "excitable media" such as cardiac tissue, neural networks, or forest fires. Each cell can be in one of three states: Resting, Excited, or Refractory.
A Resting cell becomes Excited if it has enough Excited neighbors. An Excited cell automatically becomes Refractory (recovering), and a Refractory cell eventually returns to Resting.
This simple cyclical behavior reliably generates beautiful rotating spiral waves and concentric target patterns.