哺乳類

1. Origins, structure, and basic meaning

• Structure: 哺乳類（ほにゅうるい） means mammals, the group of animals that feed their young with milk.
• Basic usage: Use it when describing the classification of whales or animals living in a region.

2. Common pitfalls and similar expressions

• Habitat versus animal classification
  • 海の生き物 groups together creatures that live in the sea.
  • 哺乳類 is a classification based on bodily and reproductive characteristics, including nursing, rather than habitat; whales belong to it.