Joker Exchange

An action that a player may perform when it is their turn to play that allows a player to trade a natural tile from their unexposed tiles (tiles in the sloping portion of their rack) for a Joker tile exposed on either the top of their own rack or on top of any other player’s rack. 

This action must not be initiated until after the player has picked a tile from the wall or claimed a discarded tile and made a valid exposure.