"Well," you said, "I wish that you will become human with the ability to turn back to a mermaid at any time."
"You made a wish for me? Maybe humans aren't as selfish as I thought. But why give me the ability to become mermaid again at any time?" She asked.
"For whenever you feel home-sick." you said, "I won't try to stop you."
"Makes sense." she said, "Okey, I'll grant your wish."
And with that, her tail became two human legs before your very eyes. As she was trying out her new human legs, you realized that she was naked.