Baseball is a team sport played on a special field over the course of several periods of play, called innings. Each inning is further divided into two parts, the “top” and the “bottom,” during which one team tries to score points while the other team defends the field. Points are scored by hitting a thrown ball (the baseball) with a bat, and then running around part of the field and returning to the start position. If the runner is tagged with the baseball by the defensive team before completing the run, the runner is out. After three outs, the inning moves forward and the teams switch places. College and professional baseball games are played over nine innings. Less intense levels of competition usually feature six or seven. The entire defensive team is always on the field. Offensive players try to hit the ball one at a time, so to start with there's only one offensive player on the field. As play progresses, up to four offensive players may end up on the field, but only one batter is up at any given time. The other three players are simply waiting in safe spots to complete their runs and score points.