A man was walking on the sidewalk and noticed up ahead that Little Johnny was wearing a red fireman's hat and sitting in a red wagon. It appeared that the wagon was being pulled very slowly by a large German Shepherd. When he got closer to the lad, he noticed that Little 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, sure" Little Johnny replied, "but then I wouldn't have a siren."