These three underage girls

These three underage girls (blonde, brunette and redhead) walk into a bar. 
A cop walks in and recongizes them and knows they were all underage. 
As he approached the girls, they all bolted. 

They ran into an alley where there were three trash bags. With the cop 
coming, they quickly hid in the bags. The officer comes looking and 
kicks the first bag (where the brunette was) and she says "meow" and 
the officer says "oh it's just a stupid cat". 

Then he kicks the next bag (where the redhead girl was) and she 
says "woof woof" and the officer says "it's only a stupid alley dog". 

Then he kicks the last bag (where the blonde is hiding) and she says 
"potato's potato's."