Mammals and animals

Today, Avery and I had a conversation about different classifications of animals. I was explaining to her that mammals are warm blooded, carry their babies in their bellies instead of laying eggs, and nurse their babies. She was still a little confused, and kept thinking that all animals are mammals, so I tried to teach her a little more about animals that are not mammals. Our talk went something like this:
Me: If an animal has fur, warm blood, carries its baby in its belly and nurses it, what is it.
Avery: A mammal?
Me: Right, and if it has feathers and lays eggs and flies in the sky, what is it?
Avery: A bird.
Me: Good! So if it has fins and scales and lives in the ocean, what is it?
Avery: A MERMAID!