If an eligible receiver on the passing team legally catches the ball, the pass is completed and the receiver may attempt to advance the ball. If an opposing player legally catches the ball (all defensive players are eligible receivers) it is an interception. That player's team immediately gains possession of the ball and he may attempt to advance the ball toward his opponent's goal. If no player is able to legally catch the ball it is an incomplete pass and the ball becomes dead the moment it touches the ground. It will then be returned to the original line of scrimmage for the next down. If any player interferes with an eligible receiver's ability to catch the ball it is pass interference which draws a penalty of varying degree (largely depending upon the particular league's rules).