The little girl and her fire truck

A fireman is at the station house working outside on the fire truck when he notices a little girl next door. The little girl is in a little red wagon with little ladders hung off the side.
She is wearing a fireman's hat and has the wagon tied to a dog. The fireman says "Hey little girl. What are you doing?" The little girl says "I'm pretending to be a fireman and this is my fire truck!"
The fireman walks over to take a closer look. "Little girl that sure is a nice fire truck!" the fireman says. "Thanks mister", says the little girl. The fireman looks a little closer and notices the little girl has tied the dog to the wagon by it's testicles.
"Little girl", says the fireman, "I don't want to tell you how to run your fire truck, but if you were to tie that rope around the dog's neck I think you could go faster."
The little girl says, "You're probably right mister, but then I wouldn't have a siren!"