Jessica woke up and noticed the stone. "This thing is my younger brother's," she thought. "He got it from Grandpa with some silly story about a wish. Like he thinks I didn't know about it. How did it get in here? Did he try to sneak into my room for some reason?"
After showering and dressing she decided to take the stone. Jon would probably ask if she had it eventually. Until then, there's no need to keep cleaning up after him. And eventually, she wondered if the stone could possibly work. It was a silly idea, a magic wishing stone. But still... "I wish I had a poster of some cute guys on the wall."
Suddenly the poster appeared, of some band she never heard of but who certainly did look cute. "Whoa," she said. "Grandpa wasn't nuts after all. This thing really works. I wish...."