The Ball In and Out of Play
In order for the ball to be out of play, it must pass COMPLETELY over the line. If any part of the ball is touching the line, it is still in play.
This applies to scoring a goal also. The ball must COMPLETELY cross the goal line for a goal to be scored. In fact, the goalkeeper may stand INSIDE the goal and catch the ball before it crosses the line!
Keep this in mind when challenging a referee's call! Some plays happen very quickly and there is no instant replay!