When it's going cheep!
Did you hear the one about the crow and the telephone pole?
He wanted to make a long distance caw.
or
When it came time to pay, the skunk didn't have a scent, the deer didn't have a buck so they put the meal on the duck's bill.
or
Because the chicken was on vacation.
or
Bike helmets don't fit hippos
or
Two. One to take notes while the other screws it into the faucet.
or