Puberty is the physical process that turns a child into an adult capable of reproduction. It is a crucial developmental period, during which cognitive processes and every part of the body change and mature. This is when secondary sex characteristics appear, such as breasts for girls and facial hair for boys. It is also when girls start to menstruate, and when boys decisively outstrip their female peers in size and strength. Once people have gone through puberty their sex will almost always be discernible by others, instinctively and at a glance, even when they are fully clothed.

Puberty is a transitional period during which children stop depending on adults and start to live independently. Societies around the world mark this transition with a variety of “rites of passage”, which may involve a solitary journey, test of strength, bonding with peers and in some places painful genital procedures.

How a young person feels about puberty can vary a lot, depending on both personal and societal factors. One may welcome being seen as mature and independent; another may dread this. The arrival of periods is often difficult for girls, as menstrual blood is often regarded as dirty and even taboo.

Oestrogen is normally described as the “female sex hormone”, and testosterone as the “male sex hormone”. In fact, both are normally present in both sexes, but in very different amounts. Levels of sex hormones also vary throughout life. They are usually low in young children, and increase a lot during puberty as the ovaries or testes mature. It is this big increase that causes the very visible changes of puberty, including reproductive maturity, secondary sex characteristics and fast growth to reach eventual adult height, as well as invisible changes such as increased bone density and important changes to the brain.