Jon told his grandfather about all the stuff that happened since getting the stone, from how he tested it out on his own, then showed it to Karyn, which led to Karyn's careless wish, then later to Jon's own careless wish that turned his best friend into Santa, then everything else that followed, bringing them to the moment when Jon got his grandfather back.
"That's incredible, Jon," his grandfather said. "I never thought that giving you my stone could bring me back, but it seems that you had inadvertently found a way to do it, even if by accident."
"I'm just glad that everything worked out, that I've got you back," Jon said, smiling.
His grandfather smiled back, but then said something distressing to Jon. "I'm glad to be back. But I'm going to have to take back my stone now."
"Wait, what?"
"It is my stone, Jon. You only received it, because it was an inheritance. I'm no longer dead, so you having it makes no sense." He stood up and looked down at Jon, holding his hand out, expecting Jon to just hand it over.
But Jon liked having the stone. Would he so easily give it up, even to his own grandfather?
