The cops were after three girls: a brunette, a girl with jet-black hair, and, of course, a blonde.
The three girls were on the run and they spotted a barn. With nowhere else to turn and having the cops hot on their trail, they ran into the barn. The place looked like it had been uninhabited for quite some time. They looked around seeking a place to hide. They saw a few empty potato sacks and each girl forced herself into a different sack.
The police came running into the barn-shed, assuming that the girls had gone into it, seeing as there were no other places to hide. They saw three full potato sacks. One cop went up to the first sack that had the brunette in it, and kicked it. Thinking quickly, the brunette meowed a few times. The cop figured, well, there's just a bunch of kittens in there.
He went up to the next sack, which contained the black-haired girl, kicked it, and following the idea of the brunette, barked a few times. Again, the cop figured, well, there's just a bunch of puppies in there.
The cop went up to the last full sack, the one that was hiding the blonde, and kicked it. Feeling extremely clever, the blonde went, "Potato!"