Too slow

Jon reached for the rock but Sarah ran under the bed before he could get to her.
From under the bed she quickly made a wish. "I wish Jon was three feet tall and my twin" Jon grabbed her just as the wish took effect. As his hand closed around her ankles he heard her make the wish and managed to pry the rock from her tiny fingers. He was too late he realized as he fell over in pain as his body began to change. Just before he lost conciousness he swallowed the now pebble sized rock.