Sex is the biological classification of many living things as male or female based on their role in reproduction. In most animal species, including humans, individuals are born either male or female, and this distinction relates to how the species creates new life.

What determines an organism's sex varies remarkably. In humans and most mammals, sex is determined by chromosomes: microscopic structures inside your cells that carry genetic instructions. But other animals have different methods: in certain turtles and crocodiles, the temperature surrounding the eggs during development determines whether hatchlings will be male or female.

Many species show visible differences between males and females, called sexual dimorphism. Male peacocks display brilliant tail feathers while females wear modest brown plumage. In some species, like many spiders and deep-sea anglerfish, females are much larger than males.

This basic biological distinction is one of nature's oldest patterns, shared by the majority of animals on Earth.