Of course, robots couldn't tan, so their skin color was always exactly the same. But if Karyn had become human, her skin could change, so it would probably be lighter or darker. And there was a subtle change in the way (the) Karyn(s) moved, as if her weight had changed. So the stone must be real!
Then it registered on Jon's mind that just before Karyn(s) became human, Eric had said "Karyn, change back to human."
Jon didn't know, but Eric had wished that Karyn be a robot until he said otherwise. So in order for the stone to change Karyn to human, it had to force Eric to say otherwise.
And when Jon heard that, he began to realize something was wrong, and not just having two Karyns. "Eric, what was that all about? I thought you said you built Karyn as a robot. Why would you say 'change back to human' if she's always been a robot?"
"I meant, uh, that I wanted her to act human."
"Come on now, you know better than that. She was already acting human--at least as human as you could expect from someone who found out she's a machine. What's really going on here?"
Eric realized that that wasn't going to fool Jon and tried the direct approach. "Karyn, I order you to get me the stone." Unfortunately, since Karyn (both of them) were human now, that had no effect other than both Karyns making a rude gesture at him. Then they looked at each other and giggled a little at having the same idea.
Keith, who Jon had ignored, was eyeing the stone and finally leaped on him. He didn't manage to wrest the stone away, but the fact that he attacked Jon helped show Jon how important the stone really was. Buoyed by this realization, Jon decided to use the stone back on them. "I wish Eric and Keith would sit down and do nothing except answer my demands, until I tell them they can go back to normal. Now guys, tell me all about you and this stone."
Fortunately the stone was smart enough not to keep them from breathing or blinking as they did this. Soon Jon heard the entire confession. "Wow," he said. "Well, I'm never going to trust you again. Certainly not with the stone. Now what do I do with you?" He considered what to do but couldn't think of a good idea. Finally he decided to put it off for later.
Then he looked to the two Karyns, the original and the younger. He couldn't use the stone to kill one, so it looked like there would be two Karyns around. But then something bothered him. According to Grandpa's note, the stone changes reality. If it did, then there must be some change in reality that explains why there is a second younger Karyn around. Nobody in the room knew what the change was, since they heard the wish, but there must be something. As Jon looked around the room, he noticed the robot equipment was gone, replaced by some other strange device--there was a change in reality.
"I wish that me and both Karyns know why there are two Karyns in this reality."
