Sex is not only something we do, it is also something we are. We are a boy or a girl from before birth. That is our sex, male or female. When we come into the world, our sex is the first thing they tell about us. ‘It’s a boy’, they say. Or ‘It’s a girl’. They can tell by our sex organs.
Exceptions
Not always, of course. To everything in the world there are exceptions. Maybe one in every hundred babies is not clearly a boy or a girl. The genitals are not fully developed or are mixed up. It also happens that a baby looks most like a boy, but after many years finds herself to be a girl. Such children usually have a hard time.
Hormones
The differences between boys and girls are both inside and outside. Inside, boys and girls have, for instance, different amounts of several hormones. These are made by their sex organs. These hormones make them look and behave differently even when they are small. When they reach puberty, the differences become very clear. And when they are fully grown, most people are mainly male or female in the way they behave. Everybody also has some of the hormones of the other sex.
Different upbringing
Boys and girls have a different upbringing. They have a typical boy’s name or girl’s name. They are called ‘he’ or ‘she’ every day. They are dressed differently. They get different toys to play with. They get different presents, play different games. When girls and boys get a little older and wiser, say when they are eight or nine, they know all about the difference between the sexes. Girls say ‘Boys are rough’. Boys say ‘Girls are silly’. Not always, of course. Also, boys and girls discover very early that their sex organs are private parts, which are covered even when they go swimming or sunbathing. Still, before puberty, boys and girls are also similar in many ways. They are learning so much that is the same in school, around the house, or watching television. They make the same discoveries about sexual feelings, playing with themselves in secret. Also, there are some games which boys and girls play with each other. For more info info about this, see sex games.
Sex 
Boys and girls can have secret sweet feelings for someone special, whose favour they try to win. If they fail in this, they feel miserable. If they succeed, they ‘go’ together, but other boys and girls tease them or gossip, and so the love affair can easily come to an end again.
Most boys and girls grow up in confusion about sex. Is it good or bad? When lovers kiss in a movie, the violins play, and its very emotional. But you also hear dirty jokes and unpleasant talk. Many ‘dirty’ words are sexual, like ‘fuck’ and ‘dickhead’ and ‘bitch’ and ‘bastard’. Girls and boys can giggle or scream or boast about anything sexual. Maybe you’ve done this too. Certain things are forbidden and you are told not to do them, for instance, accept sweets from a stranger. But what what exactly is bad about that you don’t know. And you may have heard that ‘sinners’ go to hell’, but you like to play with yourself and that gives you a good feeling. By the time they reach puberty, all children have learned to discriminate between boys and girls. Most of them also have some sexual experience or other. Not always pleasant, sometimes frightening, but often exciting.