After swimming away from shore for a while the mermaid swam to the surface, stopped, and turned to you, "Something on your mind?"
"A lot of things," you responded, "I don't even know your name."
The mermaid smiled, and you decided hers was the best smile you'd ever seen, "It's Echo. What's yours?"
"I guess I'll need a new one." You thought for a moment, "The only one I can think of is too cliché." After some coaxing from Echo you tell her the name you were thinking of is, "Ariel,"
"I think it's a pretty name, Ariel. Now what did you want to know?"
"Well, now that we're not drying out, can you explain the 'finer points of telepathy'?"
"Sure. We're telepathic, but we can't know everything that's going on in someone's head because then there would be no space for anything to go on in our heads. So we have to pick something to focus on. Before, when I used it the first time, I was focusing on what you were going to do, so I knew that you planed on holding me down instead of bringing me to the sea, but I wasn't focusing on your motivations, so I didn't know you wanted me to change you. It's really useful if you know what to watch for, and pretty useless if you don't."
"And what about the costume? I saw you put it on, but you're not wearing a costume now."
She looked away for a moment. "I was captured and made to live as a human and work in that infernal store. The same sorcery that turned me into a human prevented me from escaping. Every day I'd walk by that magic costume, and I knew that it could turn me back, but I couldn't pick it up off the rack, and I couldn't put it on for myself. So when I saw you take it off the rack I took it from you and put it on as a demonstration for you. As soon as I did I was myself again. I was honestly puzzled for a moment when you mentioned a costume, because I could feel that I was just wearing my ordinary top.
"Once I was a mermaid again I could use my powers but I was still prevented from leaving the store, so I had you TAKE me out of it. I'm sorry I threatened you, but I didn't know if those loopholes would ever open again and I had to get out of there. I'm truly grateful that you saved me. I don't think you can understand how grateful I am. So if you want to be a mermaid, I'll do everything in my power to make you the happiest mermaid around.
"Now if you want to be a mermaid there's a lot you need to learn. How to talk underwater for one thing. English doesn't work very well so you'll have to learn a whole knew language. You can take your time though, mermaids don't age so we have all the time in the world."