Identities and Spaces
Gender is a complicated subject.
The simplest possible view of gender is that there are two kinds of people, men and women. They can be identified by their external characteristics: girls have long hair and wear pink, boys have short hair and wear blue. If you want to make it sound more scientific, you can point out that women are, by and large, more emotionally connected and empathic, and boys are stronger and physically larger.
Sooner or later, you have to confront the existence of people who do not fit into those categories. These people will often be incorrectly identified as one gender when they are not. One solution to this is to ask each person what gender they are, and trust them to know which one they actually fit.