INFIELD GROUND BALLS
Ground balls that bound between home and first, or home and third are a little tricky. The ball is not ruled fair or foul, until the ball comes to a stop, is touched by a fielder, or goes past first or third base.
The ball may zig-zag, back-and-forth, between fair and foul ground an unlimited number of times. The ruling of fair or foul is not made until the ball finally comes to a stop, or is touched, or goes past first or third base.
THE ACTUAL RULE from the rule book
A FAIR BALL is a batted ball that settles on fair ground between home and first base, or between home and third base, or that is on or over fair territory when bounding to the outfield past first or third base, or that touches first, second or third base, or that first falls on fair territory on or beyond first base or third base, or that, while on or over fair territory touches the person of an umpire or player, or that, while over fair territory, passes out of the playing field in flight.
A fair fly shall be judged according to the relative position of the ball and the foul line, including the foul pole, and not as to whether the fielder is on fair or foul territory at the time he touches the ball. If a fly ball lands in the infield between home and first base, or home and third base, and then bounces to foul territory without touching a player or umpire and before passing first or third base, it is a foul ball; or if the ball settles on foul territory or is touched by a player on foul territory, it is a foul ball. If a fly ball lands on or beyond first or third base and then bounces to foul territory, it is a fair hit.