He had to do something, he had this awesome power over people, he couldn't let it go to waste. Jeff thought about what he could do. He could just go and change her to be nicer, but that wouldn't as satisfying as seeing her embarrassed that she has changed into whatever he wanted. What could he do? What did he want to do? Jeff continued to think, and think, and think. Finally, something struck his mind. Jeff always had an interest in animals. Their biology, their behaviours, just anything that made them them. This made Jeff think about how that would translate if they were like him, a human. What would change? How would they act? Would their thoughts be the same or would they be civilised people? Who knows?
Jeff did...
...or at least he would. His sister was the perfect choice to see what it would be like if animals evolved like humans. All he had to think about was what animal he would change his sister into...