For me it is Cellular Automata, and more precisely the Game of Life.

Imagine a giant Excel spreadsheet where the cells are randomly chosen to be either "alive" or "dead". Each cell then follows a handful of simple rules.

For example, if a cell is "alive" but has less than 2 "alive" neighbors it "dies" by under-population. If the cell is "alive" and has more than three "alive" neighbors it "dies" from over-population, etc.

Then you sit back and just watch things play out. It turns out that these basic rules at the individual level lead to incredibly complex behaviors at the community level when you zoom out.

It kinda, sorta, maybe resembles... life.

There is colonization, reproduction, evolution, and sometimes even space flight!