If you can't see the diagram to the right then you need to install Flash 5 by clicking below.

Puberty starts when the pituitary gland begins releasing hormones into the blood which tell the sex organs to start making sex hormones. The pituitary is a small organ, the size of a pea, in the middle of your head just underneath your brain. It also makes human growth hormone, which helps cause the growth spurt during puberty.

In boys, the testicles (also called testes) start making the male sex hormone testosterone. This is what causes most of the changes during puberty.

In girls, the ovaries start making oestrogen (pronounced 'east-row-jin') and progesterone. Oestrogen is the female sex hormone that causes most of the changes during puberty. Progesterone, oestrogen and other hormones work together to control the menstrual cycle.

Hold your cursor over the pituitary gland to see what happens.

Surprisingly, men's bodies also make a small amount of oestrogen and women's bodies make a small amount of testosterone.