Your mom is so stupid, she got locked in a bathroom and pissed in her pants.
Your mom is so stupid, she tripped over a cordless phone.
Your mom is so stupid, she got hit by a parked car.
Your mom is so stupid, she spent hours to find the "any" key on the keyboard.
Yo Mama's so fat she killed the dinosaurs
Your mom is so old that she owes Jesus a quarter.