Friend, your mum's fat.
Me: Well, your mum's so fat, she played pool with the planets.
Friend, your mum's fat.
Me: Well, your mum's so fat, she played pool with the planets.
Okay, so basically, the joke is all about how fat someone's mom is. The punchline is that she's so massive that she can use the planets as pool balls. Like you, I guess you wouldn't get it. Your brain is too smooth to understand big brain jokes.