A man was walking on the sidewalk and noticed up ahead that Little Johnny was wearing a red fire man's hat and sitting in a red wagon. It appeared that the wagon was being pulled slowly by a large Labrador Retriever.
When he got closer to the lad, he noticed that Johnny had a rope tied around the dog's testicles, which probably accounted for why the dog was walking so gingerly.
Smiling, he spoke to the little boy, "That's really a nice fire engine you have there son. But I'll bet the dog would pull you faster if you tied that rope around his neck."
"Yeah," Johnny replied, "but then I wouldn't have a siren."