If an opponent has exposed a Joker standing in for a real tile, you may swap in that real tile from your own hand and take the Joker for yourself.
This is one of the more advanced rules in American Mah Jongg: you trade the actual tile the Joker represents for the Joker itself, placing your real tile into the opponent's exposed set and adding their Joker to your own hand.
Example
If an opponent's exposed Pung is two real 6 Bams plus a Joker, and you're holding a third 6 Bam that's otherwise useless to you, you can swap it in and take their Joker.
