Can't undo wishes

I looked around in confusion. "Why haven't things changed?" I asked in confusion. "'Cause ya can't unwish a wish Johnny," grinned Mikey, striding into my room. He shut the door behind him and hopped up onto my bed. "Huh?" I asked, even more confused. "The rules of the stone," he said, gesturing for emphasis, "you did read them right kiddo?" "Kiddo?" I snorted, looking down at him. "It's simple," he sighed "you can't make that wish 'cause I already did!" I stared at him in shock. "I used to be the older brother," he explained "Michael or Mike in a pinch. You were little Johnny. I wished to trade lives with you and..." He shrugged with a grin.