Suppose you offered free gambling tips for a limited amount of time. People call in a number and are advised to bet for or against a given team. The catch: the hotline tells half the people to bet on one team and half on the other. So if 10,000 people call, then 5,000 will be winners. You can offer them the chance to get another tip. The same thing happens, and half of them are winners, or 2,500. That quarter or so of the original group starts to think this tipline is something special. Then by the time you whittle it down to people who have won six bets in a row or so, they think this gambling advice is golden and will be willing to pay for the previously free tips. They will buy a subscription to your service.
No matter which team wins, half the people will win their bet any given week. People who bet on contests where the odds are against them will feel especially lucky.