Orphanage kid: You’re ugly!
Kid with mother: Your mom!
Orphanage kid: You’re ugly!
Kid with mother: Your mom!
Listen up, buttercup. See, the joke is that the orphan kid insults another kid. Then, the kid with a mom claps back with "your mom"... except the orphan doesn't HAVE a mom! It's funny 'cause it's mean. You wouldn't get it, though.