h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

If a ball hits the ground in fair territory and rolls across the foul line BEFORE it passes first or third base (without having been touched by a player since being hit), it is a foul ball.

If a ball hits the ground in fair territory and rolls across the foul line AFTER it passes first or third base, it is a fair ball.

A ball can also become fair if it rolls back into fair territory before it has made contact with any player.

And don't forget that if a ball (prior to reaching the base) is touched by a fielder on --- or if, in the outfield, the ball hits on --- the foul line, it is fair. Same with home runs hitting the foul pole. Which begs the question as to why it's not called the fair line/pole.....or why a ball missed/not hit but in the zone is called a strike, a word normally thought of as "to hit"....or a poor pitch is called a ball, which is what they're supposed to go after....or why the coaches dress in unis like the players.....

I vaguely recall a game on TV years ago where the batter layed down a beautiful bunt along the third base line. The third baseman had no play, so he got down on all fours and tried to stare the slow rolling ball foul. the announcers suspected he was blowing on the ball.