Here mainly refers to "inclusion", that is, in the category of preschool children's mathematics education, the set and the elements in the set are the relationship between inclusion and inclusion. It is different from the relationship of "belonging" in a set. "Ownership" refers to the relationship between elements and sets, and "inclusion" refers to the relationship between sets, and the two cannot be confused.

Characteristics of the set:

A sure thing

Given a set, any element, whether it belongs to the set or not, must be one of them, and there is no ambiguity.

Mutual anisotropy

Any two elements in a collection are considered different, that is, each element can only appear once. Sometimes it is necessary to describe the situation where the same element appears many times. You can use multiset, where elements are allowed to appear multiple times.

randomness

In a set, the state of each element is the same and the elements are out of order. You can define an order relation on the set. After defining the order relation, you can sort the elements according to the order relation. But as far as the characteristics of the set itself are concerned, there is no necessary order between elements.