This is a simulation of an excitable medium, using the summed-area table algorithm (thank you to Dave Greene for reminding me of the name). Click on the simulation area below to add a square region of excited cells. If you spam click randomly eventually you'll get a self-perpetuating spiral.
It's kind of mesmerising to watch. I hope you have fun playing around with the parameters.
Threshold:
Radius:
Refractory time: