You can't see the injury, but concussions can leave behind effects bigger than the blow that caused them. It happens when the brain moves around in your head and hits the walls of your skull.
According to the Centers for Disease Control and Prevention, 7 out of 10 concussions go undiagnosed, because people don't know what signs to look for.
FULL COVERAGE: Concussions: A Real-Life Penalty
A referee throws a flag to signal something that went wrong during a play, but what if the ref didn't know all of the rules? He may miss a call that could blow the game.
Concussions are like that. Missing one of the signs could end an athlete's season, or even life.
According to the Centers for Disease Control and Prevention, 7 out of 10 concussions go undiagnosed because people don't know what signs to look for.
The CDC estimates there are 3.9 million concussions a year. Noah Johnson was one of them.
He says, "When I first got the initial hit, I blacked out. I don't remember anything from when I blacked out to when I came to whenever I was standing up by the coach after being carried back by the boy that hit me."
He falls in the less than ten percent of athletes who actually lose consciousness. The Jones County freshman says even though he felt fine afterward, his brain wasn't.
"A concussion, you gotta watch it because sometimes you may look alright, but you really ain't. That's the way I was, because I looked totally normal, fine but if I did any physical activity, started playing around, running around, throwing a football or anything, I'd get a real massive headache," says Johnson.
47% of athletes are like Johnson and don't feel any symptoms after a concussive blow, but medical experts say it could take up to three days for things like fogginess, mood changes, or sensitivity to light to set in. For Johnson, it happened the next day at school.
He says, "It was a little bit hard for me to focus with my headache and I'd get sidetracked."
Two weeks after the hit, Johnson still gets the same headaches. After playing football since a young age, he's eager to get back on the field.
"But it could cause me to have permanent brain damage, possibly a death if I were to keep playing or lying to them about my symptoms, and if I took another hard blow to the head, I could have permanent brain damage or be paralyzed," he says.
Three out of four concussions happen during a game or competition, according to the Sports Concussion Institute, but Johnson's knockout during practice taught him that every hit, every play could lead to a devastating life penalty if you don't know the signs and when to throw the flag.
"It really opens your eyes."