What's in the bag?

A redhead, a brunette, and a blonde robbed a supermarket. As they were stealing, a police officer walked in the store and saw what was happening. He dashed toward them, but they were able to get away into the back of the store. There they found three sacks to hide in. When the police officer checked there, he examined each sack.

He kicks the first bag, and the redhead says "meow" in a high voice. The cop determines that it must only be a cat in that bag, and he moves on to the next.

When he kicks the second bag, the brunette says "woof" in a low voice. The officer determines that it must only be a dog in that bag, so he moves on to the last bag.

He kicks the third bag, and the blonde shouts "potato" to the officer.