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!
A fact I’ve recently enjoyed spreading around: all of humanity’s radio communications have traveled about 200 light years from Earth. The diameter of the Milky Way galaxy is ~100K light years. So (in the worst case) we’re like 0.2% of the way to even being a “blip on the radar” of any alien life within our galaxy.
Also interesting is that because the energy of those signals is spreading out as they move away from their point of origin they become less detectable as they travel. Most signals would fall below practical detection limits before making it halfway to the nearest star. At the extreme, the Arecibo Message, transmitted with a ridiculous ERP, will be detectable to reasonably sized receivers for tens of thousands of light years, assuming they are located along the path of the beam.