Conversation so far:
where must a throw in be taken from
The throw-in is taken from the point where the ball crossed the touch-line, either on the ground or in the air.
what sports are throw-ins from
A throw-in is a method of restarting play in a game of football (or soccer) when the ball has exited the side of the field of play.
what kind of ball is used in soccer
The standard soccer ball is made of synthetic leather, usually polyurethane or polyvinyl chloride, stitched around an inflated rubber or rubber-like bladder.
what were soccer balls made of before synthetic leather
Older soccer balls were made of genuine leather and held shut with cotton laces.

Question: when did the balls become synthetic
Self-contained rewrite: when did soccer balls become synthetic instead of leather