.. the Obvious Proximity Stone.
Throughout its long history (which may be longer than Jon's grandfather may have led Jon to believe), the wishing stone had caused the creation of several different variations of it, each at least slightly different from the regular wishing stone. And many of these fell into the hands of people who wanted to find the original wishing stone and switch it with the variant they had possessed (for reasons of their own).
These other stones, too, inspired variations. And some people had created variations that were combinations of two or more of the different stones.
The Obvious Stone worked just like the regular wishing stone, except that everyone would notice the changes in reality regardless of whether or not they heard the wish. For example, let's say Jon wished all the trees in the town of Lake Point would turn blue to match the blue branch created by his previous wish. If he made that wish with the regular wishing stone, everyone who didn't hear the wish would think all the trees were always blue, but if Jon made that wish with the Obvious Stone, then everyone in town who didn't hear the wish would be wondering why all the trees suddenly turned blue.
The Proximity Stone worked just like the regular wishing stone, except that one didn't have to be holding the stone for their wishes to be granted. As long as the person was within 20 feet of the stone when they made their wish, their wish would be granted whether they knew the stone was there or not.
At some point in history, someone had the idea to create a stone that was a combination of both of these stones: The Obvious Proximity Stone.
Therefore, the Obvious Proximity Stone would grant wishes made by anyone within 20 feet of it, and everyone would notice the changes made whether they heard the wish or not.
And Jon just so happened to have been visited by the Stone Switcher who had this particular combination variant, and switched the regular stone with the Obvious Proximity Stone.
Jon would soon wake up the following morning. How long would it be before he or someone else made a wish?
