Two feet: chicken, duckling and bird.

Four feet: rabbit, frog and kitten.

Method 2: Divide by category.

Birds: chickens, ducklings, birds.

Others: rabbits, frogs and kittens.

Classification refers to classification according to category, grade or nature.

1. 【 classification; Classify according to category, grade, or nature.

2. Divide irregular things into regular ones, and separate things according to different characteristics to make things more regular.

3. The establishment of the classification system of biological categories is based on the assumption that there is a natural relationship between organisms. The science of biological classification is generally called taxonomy.