Oink, Moo, Potatoes.

A blonde, a brunette, and a red head were on the run from the cops.  They ran into a barn, and decided it was safe to hide there. 

The brunette hides with a group of pigs, and the redhead hides with a group of cows. 

There were no more animals, so the blonde hides behind a sack of potatoes.

The cops see the farmer and asks if they can search the barn, and the farmer said to go right on ahead.

The cops walk in, and stop at the group of pigs. The brunette goes "Oink, oink, oink."  The cops find nothing strange, and walk along.

They stop in front of the group of cows, and the red head goes "Moo, moo, moo."  The cops, again, finding nothing strange and walk ahead.

They stop in front of the sack of potatoes, and the blonde calls out "Potato, potato, potato!"