When Manning entered the NFL Draft (which is where professional American football teams choose new players for their teams), he was first chosen by the San Diego Chargers. But, the same day that the San Diego Chargers chose Manning, they decided to allow him to play for the New York Giants. In return, the New York Giants allowed Philip Rivers to join the San Diego Chargers. The New York Giants also allowed the San Diego Chargers to choose more players the next time the NFL Draft happened.