Here's why:
* Animals is a very broad category. Fish are definitely animals, as they are multicellular organisms that obtain nutrition from other sources (heterotrophs).
* Mammals are a specific class of animals. They share characteristics like having fur or hair, giving birth to live young, and nursing their offspring with milk. Fish do not have these traits.
Fish belong to a different class of animals called Pisces. They have gills to breathe underwater, fins for swimming, and scales covering their bodies.