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.