Here's a thought....
If the hand is part of the ball, and B1 slaps A1's hand while on the ball, causing the ball to go out of bounds....
Would it not be A's ball - because if B1 touched A1's hand while it was on the ball, it was the same as touching the ball (because the rule says the hand is part of the ball), therefore, technically, B1 was last to touch the ball - so the ball should go to Team A.
Oh, and one more thing....who's on first?
