Two couples are playing cards. John accidentally drops some cards on the floor. When he bends down under the table to pick them up, he notices that Bill's wife isn't wearing any underwear.
Later, John goes into the kitchen to get some refreshments. Bill's wife follows him and asks, "Did you see anything that you liked under there?"
John admits that he did. She says, "You can have it, but it will cost you $100."
They decide that John should come to her house around 2 p.m. on Friday while Bill is at work.
On Friday, John arrives at 2 p.m. He pays Bill's wife $100. They go to the bedroom, have sex and then John leaves.
When Bill comes home at 6 p.m., he asks his wife, "Did John come by this afternoon?"
Reluctantly, she replies, "Yes, he did stop by for a few minutes."
Next Bill asks, "Did he give you $100?"
She thinks, "Oh hell, he knows!" Finally she says, "Well, yes, he did give me $100."
"Good," Bill says. "John came by the office this morning and borrowed $100 from me. He said that he would stop by our house on his way home and pay me back."