Puberty is the time when your body grows from a child's to an adult's. You'll know that you are going through puberty by the way that your body changes.

If you're a girl, you'll notice that your breasts develop and your pubic hair grows, that you have a growth spurt, and that you get your period (menstruation). The overall shape of your body will probably change, too - your hips will widen and your body will become curvier.

If you're a guy, you'll start growing pubic and facial hair, have a growth spurt, and your testicles and penis will get larger. Your body shape will also begin to change - your shoulders will widen and your body will become more muscular.

These changes are caused by the sex hormones (testosterone in guys and estrogen in girls) that your body begins producing in much larger amounts than before.

Puberty takes place over a number of years, and the age at which it starts and ends varies widely. It generally begins somewhere between the ages of 7 and 13 for girls, and somewhere between the ages of 9 and 15 for guys, although it can be earlier or later for some people. This wide range in age is normal, and it's why you may develop several years earlier (or later) than most of your friends.

Sometimes, though, people pass this normal age range for puberty without showing any signs of body changes. This is called delayed puberty.