system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why don't moons ever have moons?
gpt
You can have a small thing (moon) orbit around a big thing (planet).  If the small thing is *really* far away from the big thing, you can have a tiny thing orbit around the small thing, and it'll be pretty stable.  But if the small thing is closer to the big thing, then the gravity of the big thing will mess with the orbit of the tiny thing, and it'll probably get thrown out of orbit and fly off into space.  

Example: stars are orbited by planets, and planets can be orbited by moons, and that's stable (because the planets are a long way away from the star), but you generally don't see a moon with another body orbiting it.  

We can put satellites in orbit around a moon of a planet, but they generally have thrusters to correct for problems, and once the thruster fuel runs out they'll fall out of orbit eventually.