The human body grows from its conception until its late teens. And this growth is irregular. The first few years and the early teens are the times of most rapid growth. Baby boys commonly grow faster than girls, but then girls outstrip boys until the age of four, at which point they both grow at around the same speed.
Often, girls reach puberty first and are taller than boys for a while, but when the testes release testosterone at puberty, boys can grow up to 12cm (5in) in a single year.
Reproductive or sexual organs are there from from birth, but it is only at puberty - typically between 11 and 13 - that these are switched on by hormones from the pituitary gland in the brain. Girls develop breasts, pubic hair and, two years later, underarm hair. Their hips broaden, their waists slim and their ovaries grow up to 10 times their original size, releasing the hormones oestrogen and progesterone. The menarche (first period) is followed by erratic periods, but these settle down to a cycle of about 28 days.
Female sex cells (unlike male sperm) are there from birth. Each cell is held in a follicle in the ovary. There may be half a million follicles in the ovary, but only a few become eggs. Each month, the pituitary gland in the brain sends out follicle-stimulating hormone (FSH) and a few dormant follicles start to grow and secrete the sex hormone oestrogen.
This stimulates the growth of the lining of the uterus. This endometrium is ready to accept a fertilised egg. One follicle grows bigger than the others and in a fortnight becomes a Graafian follicle. Stimulated by luteinising hormone from the pituitary gland, the follicle bursts from its sac, releasing the egg or ovum, which travels down the Fallopian tube to the uterus. The follicle left behind changes colour and becomes a corpus luteum or yellow body. It gives out oestrogen, but also progesterone, which helps to stimulate the building of the uterus lining.
If the egg is fertilised by a sperm, the corpus luteum goes on growing and producing more hormones, thickening the uterus lining to accept and nourish a fertilised egg. But if the egg is not fertilised, the corpus luteum wastes away and hormone levels drop. Pieces of uterus lining break away with blood and mucus. This is the menstrual flow, which lasts about five days. Then the pituitary releases FSH again and the cycle restarts.
Puberty in girls can occur as early as 8 or as late as 16. The main factor affecting this timing is weight: girls start having periods at a body weight of about 45kg (99lb). Once periods start, girls are unlikely to grow more than another 6cm (2.4in).
At puberty, boys develop underarm, beard and pubic hair, and their testes grow, drop and produce sperm. Boys' testes are kept cooler outside the body in the scrotal sac. They produce 2 million sperm every day. Bones can grow faster than muscles, making teenage boys gangly. Their brains have to learn to control their new, and often clumsy, bodies. Surges of testosterone can lead to acne. Boys' voices deepen. Their faces change shape and their jaws become squarer. Their chests and shoulders broaden.
Teenagers of both sexes become self-conscious at puberty, but this fades as they become more socially confident and independent.
John Stringer is a science writer and consultant