Ten-year-old Johnny was walking down the sidewalk pulling a wagon and dragging a dead flattened frog on a string behind it, when he came up to the doorstep of a whorehouse. He knocked on the door, and the madam came to answer it. When she saw him with his wagon and frog, she just stared and asked what he wanted.
Johnny said "I know what you sell in there, I have the money to buy it, and I'm not leaving 'till I get it!"
She thought she would have some fun with him, so she told him to come in. Once he got in, she told him to pick out the girl he liked.
Johnny asked, "Do any of these girls have any diseases?"
Well, the madam was somewhat surprised, but of course, she told him, "No".
Johnny replied: "Well, I heard all the men were talking about having to go to the hospital and get shots after making love with Mabel, and that is the girl I want. I have the money to pay for it."
The madam stared incredulously at him, but finally told him "Well, alright then. Mabel it is. Go upstairs and to the first room on the right."
So, Johnny headed up the stairs dragging the smashed frog behind him. About ten minutes later, he came back down the stairs, still dragging the frog. He paid the madam, and picked up his wagon and headed out the door, at which time the madam stopped him.
"Since you already knew about Mabel, why on earth would you pick the only girl in the place with a disease, instead of one of the others?" she asked Johnny.
He answered back, "Well, tonight when I get home, my mother and father are going out to a restaurant to eat, leaving me at home with the baby-sitter. When they leave, I am going to screw her, and give her the disease I just caught.
When mom and dad get home, dad will drive the baby-sitter home. On the way, he will screw her too, just like he always does, and he will catch it from her.
When dad gets back home, he and mom will go to bed, and they will make love, and mom will catch it too.
Then, in the morning when dad goes to work, the milkman will deliver the milk, and he will screw my mom, and he will catch it, and he's the son-of-a-bitch that ran over my FROG!"