How many times have you heard "Hey Ref, that ball was out"?

Most of the time it's the parent's yelling, and they are wrong.  In soccer, a ball must completely go over the line to be considered out of that area.  For example, to go completely over the touchline, the whole of the ball must cross the line.  In fact, the ball can be just outside the line and not touching it at the bottom, but still be in play.  If you draw a line from the outer edge of the touchline up into the sky and that line would touch any part of the ball, it is still in play.

FIFA Law 9:

Ball Out of Play
	The ball is out of play when: it has wholly crossed the goal line or touch line, whether on the ground or in the air play has been stopped by the referee.

Ball In Play
	The ball is in play at all other times, including when: it rebounds from a goalpost, crossbar or corner flagpost and remains in the field of play; it rebounds from either the referee or an assistant referee when they are on the field of play.