A fire-fighter is on a picket at the station when a little girl comes up to give him a cake.  She's in a little red wagon with little ladders hung off the side and a garden hose tightly coiled in the middle. The girl is wearing a fire-fighter's helmet and has the wagon tied to a dog and a cat.

"That's a nice fire engine," he says.
"Thanks," the girl says.

The fire-fighter looks a little closer and notices the girl has tied the wagon to the dog's collar and to the cat's testicles.
"Little friend," he says, "I don't want to tell you how to run your fire engine, but if you were to tie that rope around the cat's collar, I think it could pull more."

She replied, "You're probably right, but then I wouldn't have a siren."