Mermaids?

So, my 7 year old little sister wanted me to blog about mermaids. So, here I go... What am I supposed to say? This is a lot harder than I thought.
Hmmm.
Mermaids.
Underwater people.
People that live underwater...
I have nothing.
Ariel was a mermaid. She had a beautiful voice... So, I conclude that mermaids all have pretty voices. Unless, its those mermaid things from Harry Potter. Those are creepy. And they sound like cats that want to be put down because you were carrying them upside down...Maybe that is a slightly strange analogy.
So, are mermaids like dolphins? Do they need to breathe air? Or do they have gills? I'm really curious now. I would like to meet a mermaid at some point. I would ask it what kind of currency they use. Or if they have currency.
And do all memaids have extremely long hair? How do they brush it out? So many mermaid related questions.
Can mermaids read? Or like, write? Idk. And why do mermaids supposably save drowning people? Are they like super territorial? Like, "You don't belong in the water. Get out!" We may never be able to answer these questions.