This fat guy sees an ad for a new gym guaranteeing to reduce anyone's weight by 5, 10 or 20 kilograms on the first day. So he goes and tells them he wants to lose 5 kg. They strip him and lead him into a huge gym with all kinds of ropes and parallel bars and ladders and tell him to wait a minute. He's standing there when on the far side of the gym a door opens and out steps a gorgeous blonde, stark naked, with a sign saying

"If you catch me, I'm yours."

He starts running, and just as he gets close, she starts picking up speed. Before he knows it, he's running all over the gym, up the ladders, down the ladders, across the parallel bars, here and there. And just as he's about to catch the blonde, pop, she disappears through a door. In comes the management who lead him to the showers, and then weigh him. Sure enough, he lost exactly 5 kg.

He's back on the street and starts to think.

"Jesus, I was so close to catching her. If I had a little more time..."

So he races back to the gym and says, "I want to lose 20 more kg."

"No problem," says the manager.

Again he strips, and is led to the large gym. This time he's standing by the door when it opens. Out comes a gorilla with a sign

"If I catch you, you're mine."