Here's why:

* Chromosomes: These are thread-like structures that contain genetic information (DNA). Humans have 23 pairs of chromosomes, for a total of 46.

* Sex Chromosomes: One pair of chromosomes determines a person's sex. These are called the X and Y chromosomes.

* Females: Females have two X chromosomes (XX).

* Males: Males have one X and one Y chromosome (XY).

It's important to note that this is a simplified explanation. There are complex biological factors that influence sex determination, and there are also individuals who are intersex, meaning they have a combination of chromosomes or physical characteristics that don't fit typical male or female definitions.