Can a person become a mermaid?

First off this is for my little sister. She loves H2o and Little Mermaid and that other stuff. Now I'm not a dream crusher and I just want to know, is it even POSSIBLE for a human to become a person. Please NO rude or mean comments, my little sister is only 6 years old. I don't want to hear, "mermaids don't exist" because how the heck do you know or "you need to get help" because that's just rude. I just want to know are there any potions or charms or spells a person can do? Even a location. Just want to know.