"There is no way you can fit in there."
"Says who?"
"Your mom."
"When?"
"Last night."
"OHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH-"
Alright, listen up, you silly goose! The joke is that someone's mom was with someone else *last night.* Basically, the joke is that the mom is sleeping around. I bet you didn't even know that!