So there's this really cool cellular automata (or, simulated basic 'life') thingy called Reaction Diffusion, which unlike certain other things with 'diffusion' in the name is actually really fucking cool.
Basically, there are two "chemicals" which react with one another; one particle of type A reacts with two particles of type B, which forms three particles of type B. This can of course vary with implementation; you can totally add a couple more particle types to the mix.
Particles are added and removed at specific rates, and it is purely these addition and removal rates, denoted with an F (feedrate) and k (kill rate) which dictate how the particles end up interacting. There are many different patterns possible, ie. waves moving across the screen, or little spirals that create other particles in their wake.
Patterns can grow and then stabilize after a while, or they can keep going forever. There's even certain combinations of F and k where it makes little cells that float around and undergo mitosis, or where little blobs with a winged shape spiral forth and create more spiral creatures in their wake.
Tiny changes to the parameters can create entirely different patterns.
(Here's a page with more details on how it works!)
See for yourself!
This is a pretty simple reaction diffusion simulator that I found.
I would personally recommend copying the following code, pasting it in the text window and hitting enter, as it makes the colors a bit nicer to look at (and its brightness linearly scales with the intensity, unlike rainbow colors!)
0.0323,0.0549,0,#440154,0.1,#414487,0.2,#2A788E,0.3,#22A884,0.4,#7AD151,0.5,#FDE725
(there's a bug where the last color doesn't get added, so you may need to add the yellowish color #FDE725 yourself by clicking the last color tab, entering the hex code and hitting enter; then dragging the color tab to about the midpoint of the gradient).