Our bodies change quickly during teenage years. But that's not all.
Our feelings change too, and we have to learn to look out for ourselves in all kinds of ways.
It's all part of becoming an adult.
Girls get breasts, bigger hips and start having periods, boys get a deeper voice and start making sperm - and everyone gets hairy!
They're all signs we're turning into adults and that our bodies are getting ready so they can start making babies.
But just because our bodies are able to get pregnant - or get someone pregnant - doesn't mean we're really ready to start having sex.
'Sex' can include vaginal, oral and anal sex, as well as activities like mutual masturbation, sexting, phone and cyber sex.
It's OK to choose whatever you want to do and whatever you're comfortable with, whether that's no sex at all or safer sex with your partner.
How we feel can change a lot in a short space of time while we're growing up.
It's partly down to changes in our hormones, the natural chemicals our glands produce to tell our bodies how to change and grow.
It's also because we're trying figure out our place in the world and what we want.
Learning how to cope with strong feelings is an important skill for life.
For tips and advice on dealing with all kinds of feelings, check out the 'Mind' section at Get The Lowdown.
Another important area is how we feel about people around us and about our relationships.
Growing means learning how to look after our bodies and our feelings. It means learning to take responsibility for ourselves and about how our choices affect those we love.