This one is a bit too obvious...
An overweight guy was watching TV. A commercial came on for a guaranteed weight loss of 10 pounds in a week. So the guy, thinking 'what the hell', signed up for it.
Next morning an incredibly beautiful woman was standing at his door in nothing but a pair of running shoes and a sign about her neck that read, "If you can catch me, you can have me."
As soon as he saw her, she ran off. He tried to catch her, but was unsuccessful. This continued for a week, at the end of which, the man had lost 10 pounds.
After this he tried the next weight loss plan, 15 pounds in a week. The next morning an even more beautiful woman was standing at the door, in similar conditions. The same happened with her as the first woman, except he almost caught her. This continued for a week, at the end of which he, as promised, weighed 15 pounds less.
Excited about this success, he decided to do the ultimate programme. Before signing up, he was required to sign a waiver and was warned about the intensity of this plan. He signed up.
The next morning, waiting at the door, was a hulking 300 pound muscle man with nothing but a pair of running shoes, a raging erection, and a sign around his neck that says, "If I catch you, you're mine!" ...