The Upper Hand

Looking down at the tiny near-naked woman in front of him, John realized that the rock had shrunk right along with her. Through her terrified high-pitched shouts, John managed to realize the situation just in time to beat her to the punch.
John shot his hand out and grabbed the doll-sized Sarah, cupping his hand around her proportionately large chest.
"Try to make a wish," John said, angrily staring at her as he lifted her in front of his face, "and I'll squeeze before you can get it out."
Tears streaked down her hysterical face, but Sarah said nothing, only nodded.
"Good," John smiled, "Now drop the rock!"
Sarah hesitated for an instant, but did as he said; She let the rock fall out of her hand to the floor.
The instant the rock started to fall, John grabbed it out of the air. It was now tiny, only about a centimeter long. He clutched it in his palm.
"I'm sorry! I'm sorry!" Sarah sobbed, with tears running down her face, "Please change me back!"
"Now why would I go and do a thing like that, " John stared at her, "So you can steal my rock again?"
Having loosened his grip, Sarah was on her knees begging in the palm of John's hand.
"Please! You can't do this me!"
"You forget Sarah, I didn't do this. You did. I'm not even sure I can grow you back, even if I wanted to. You can't undo wishes."
Hearing this, Sarah once more burst into tears. John did feel sorry for her, though the reason why was beyond him. But maybe she still needed to learn a lesson...