TIL that when a moon has a moon, it's called a moonmoon. Theoretical candidates that could host a moonmoon are Earth's moon, Callisto, Titan, and Iapetus, though none have ever been detected

https://www.science.org/content/article/moons-moons-could-exist-and-scientists-call-them-moonmoons#:~:text=Although%20a%20moonmoon%20has%20yet,Saturn's%20moons%20Titan%20and%20Iapetus.
#til #todayilearned
https://www.reddit.com/r/todayilearned/comments/1gt10zw/til_that_when_a_moon_has_a_moon_its_called_a/