trick plays in baseball

Question
base runners on first and second, infield, pitcher, catcher come together at the mound. once they break up and take positions, some how the first baseman has the ball, when the base runner on first leads off, the first baseman tags him. The umpire calls the runner out. Is this a legal play?

Answer
It is a legal play as long as the pitcher is not on the mound. The mound is defined as a 4-foot radius around the rubber. A lot of people have yelled at me for calling this legal but as long as the rules are followed then it is ok.

If the pitcher doesn't have the ball and comes within 4-feet of the rubber then it is a balk.