You look around you and see that most of the jobs that have high status are occupied by men.
You see on TV that most of the lead parts, are men (although not so bad now). Still, the male detective solves the crime with his genius and the female boss might be clever but is always a bit stroppy and never popular. There may be a few female scientists on TV but usually the show is about the genius social misfits who are all boys.
You learn that sport is very important and that boys are better at it. Of course girls can have a go but they won't be good enough to beat the boys. And being the best is important.
You see the history books are full of what men did. You ask what women did and get told not much, just bringing up children. You don't get told until later that it was because women were stopped in a variety of ways so think it was probably because they were a bit rubbish. You learn that women's lives couldn't have been that interesting.
Someone, a family member who you think is right about everything, tells you that boys are better at maths because they are more logical.
Your brother chucks you off his PlayStation because it's not for girls and you suspect he's right because the games are mostly about fighting and a bit boring anyway.
You hear phrases like "don't be a girl" and "man up" and get the impression that people find girls "catty" and "bitchy".
So you form the overall impression that yes whilst girls may work hard boys have a natural genius.
And if you get a bit annoyed about it you are seen as too sensitive and told that nothing is stopping you. So really it's down to you.