This was his last chance. Jon had to get that stone back.
While Biff thought about what to say, Jon lurched forward and tried to grab the stone. Biff tried to stop him, but failed.
"Now, Jon. Don't ..."
But Jon didn't let him finish. "I'm gonna give you a little of your own medicine." Biff looked at him with shocked eyes, as Jon said "I wish that you can't hear my next wish." The stone glowed.
Then before Biff could react, Jon said "I wish ..."
