Why do horror films love a girl's first period?

The full narration of the video.

People say horror movies always open with a girl's first period. That's only a little bit true. Not many films actually do this. Two of them are just very famous. Carrie, from 1976. The first scene is a school locker room: a girl gets her first period and thinks she's dying, and the other girls throw things at her.

Later she finds she can move things with her mind. Ginger Snaps, a Canadian film from 2000. The night a girl gets her first period, a beast bites her, and she slowly turns into a wolf. Something about half of all people go through. Why make it the start of something monstrous?

First answer: there's blood. In horror, blood usually means someone got hurt. But here there's no wound. The blood just comes. Nobody warned her. She has no idea what's happening. The second answer is older: people in many places feared it, for a very long time.

Nearly two thousand years ago, a Roman scholar wrote that a woman on her period could sour new wine and dull a mirror. None of it was true. But the fear was handed down. Not until 2017 did a British pad ad swap the blue test liquid for red.

The more something is kept quiet, the easier it is to make it scary. The third answer matters most: your body is changing, and you don't get a say. Think of a caterpillar becoming a butterfly. Outside, it's just a cocoon. Inside, its body nearly melts down, then rebuilds.

Film those days inside the cocoon, and you'd have a horror movie. Girls growing up are like that too. Over a few years, height, shape and mood all change. The first period is the moment you can see. Horror blows that moment up: turning into a wolf, getting strange powers. It all means: my body won't listen to me.

Stephen King, who wrote the novel Carrie, once worked as a high school janitor and cleaned the girls' locker room. Then he read a magazine claim that girls around puberty might have telekinesis. There's no evidence for that. But the two ideas collided, and the novel was born. So what these films fear isn't periods. It's growing up while nobody tells you what's going on.

A butterfly is no longer a caterpillar. A girl who grows up is still herself. In Chinese cities, most girls get their first period around twelve. It's just one step of growing up. When did you first understand what a period was? Did you hear it from family, or from a textbook or classmates?