"I wish..." John began...but paused for a moment when he realized he had an interesting opportunity before him. Within a few minutes, John was masturbating in Sarah's body bringing her to orgasm.
"Oh, God!" John moaned, "I wish this would last forever!"
John's face was in pure shock for a moment. Then he remembered that he thankfully placed the rock down on his dresser before starting to feel himself up, but there was a problem...his orgasm wasn't stopping.
"Oh, God!" John moaned again, "Why is this...uh...happening to me? Uhhh!"
John grabbed the rock from his dresser. "I wish I knew...ooooh...why I am still orgasming...uh! Yes!"
John then realized he never finished his earlier wish until he started orgasming. The rock was waiting for it, expecting it.
John had to think of some way to reverse the wish.