your mom's so stupid she put 25 cents in the parking meter, and waited for gum balls to fall out.
your mom's so stupid she put the TV dinner in the VCR.
your mom's so stupid she tripped on a wireless telephone.
your mom's so stupid she got hit by a parked car.
and my personal favorite:
your mom's so fat when she saw the school bus go by your house, she ran out and chased it cuz she thought it was a twinky.
.....dont remember where i heard them all but mostly my friends xD