Your brain does not sit tight inside your skull. It floats in a layer of fluid. Most of the time that is excellent design — the fluid cushions the small knocks of ordinary life. But it also means the brain is able to move.

A concussion happens when a hit makes it move too hard. The blow can land on your head, or it can land on your body and snap your head around. Either way the brain shifts and twists inside the skull. Cells get stretched. The chemistry inside the brain goes briefly wrong.

Here is the thing most people get wrong: you do not have to be knocked out. Most concussions happen with no blackout at all. A player can get straight back up, walk to the sideline, answer every question put to them — and still be concussed.

The effects arrive as a set of clues rather than one obvious injury. Headache. Nausea. Dizziness, or trouble with balance. Light that seems too bright and noise that seems too loud. A foggy, slowed-down feeling. Trouble remembering the play, or the lesson, or the conversation you just had. Some of it may not show up for hours — or until the next day.

A few signs mean stop everything and get to a hospital: one pupil larger than the other, vomiting again and again, a seizure, slurred speech, a headache that keeps getting worse, or someone who cannot be woken up.

And there is a hard reason coaches now take all of this so seriously. A brain that is still healing is more easily injured again, and a second hit before recovery makes everything worse and slower. That is the entire logic behind the rule you will hear shouted on every field: when in doubt, sit them out.