If the ball is hit a long distance way up in the air, it's a long fly ball. If it's hit out toward center field it's liable to be caught by the center fielder since the center field fence is farther away from home plate than the right and left field fences are. Also, a ball hit to far right or far left field will tend to curve towards foul territory. Thus a ball can appear to be going fair only to pass into foul territory before going over the fence. Good command of body english can hold the ball fair.