In professional baseball’s World Series, the championship is decided in a best-of-seven format. The first team to win four games gets the pennant. Curiously, series that go on for the full seven games appear to occur more often than simple probability arguments would suggest.
Suppose that two, evenly matched teams have made it to the finals. Each one has a 50:50 chance of winning any given game. So, for a given team, the probability for winning one game is .5, the probability of winning two games in a row is .5 x .5, or .25, and so on.