Boids
eater.net
Three rules are all a flock of birds needs
Sandboxes & SimulatorsInstantFree, no sign-up
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Sandboxes & Simulators
Four rules, and cells that build spaceships anyway
playgameoflife.com
Instant · Works anywhere
Free, no sign-up
The classic cellular automaton: each cell lives or dies based only on how many of its eight neighbours are alive. Draw a starting pattern or load known ones such as gliders, guns, oscillators and puffers, then step or run the grid and see what the rules make of it.
Loading a glider gun and realising a fixed four-rule system is manufacturing moving objects indefinitely is where cellular automata stop being a curiosity.
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Picked for a shared category and overlapping vibe, not for popularity.
eater.net
Three rules are all a flock of birds needs
pmneila.github.io
Two chemicals, two dials, and every pattern in nature
brm.io
Two dozen physics scenes you can grab and throw about
hunar4321.github.io
Coloured dots that attract and repel start forming cells
sandboxels.r74n.com
Falling sand with hundreds of elements and real chemistry
keiwan.itch.io
Build a jointed creature and let it learn to walk