system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In baseball how does the "Hidden ball" trick work, as seen on the front page today.

I assume a 5 year old probably doesn't play much baseball or any baseball at all, like me. Also, is it a 'legal' move to make?
gpt
In baseball, the ball is almost always in play.  In other words, the defense is allowed to tag out runners between plays and the offense can steal bases between pitches.  Normally when the fielders are concerned about a runner on base, the pitcher simply makes a pickoff throw, meaning that he throws the ball to a fielder covering a base, rather than throwing a pitch to the batter.

With the hidden ball trick, the fielders find some way to get the ball to a fielder covering a base without the runner knowing.  As soon as the runner steps off the base, the fielder tags the runner with the ball.  Since the runner is tagged off of a base, he's out.

Please note that there is one particular rule that makes the hidden ball trick particularly difficult.  In the MLB, the pitcher can't touch the rubber (white area on the pitcher's mound where pitches are thrown from) after giving the ball to a teammate to hide.  (Other leagues require that the pitcher can't be anywhere on the pitcher's mound.)  This makes the trick difficult because it's harder to convince the runner that the ball's in the pitcher's hands if he's not standing in the place where he would normally pitch the ball.