A knock to the head can shake the brain even when the skin looks fine. Watch for clues that start right away or build over the next day.

Headache is the common first signal. It may throb, feel tight, or grow worse when you move your eyes or stand up.

Thinking feels slow. You forget why you walked into a room, lose track of conversations, or give the same answer twice.

Stomach trouble shows up next. You may feel queasy, throw up once, or keep vomiting long after the hit.

Balance drifts. Walking turns clumsy, like weaving after spinning in place. Some people see the room spin even while lying still.

Vision can blur or double, and bright grocery-store lights hurt. Ears may ring or feel plugged, and noises seem too loud.

Sleep flips. You doze off in mid-sentence or lie awake all night and feel dazed the next day.

Mood swings fast. A calm person snaps at small things, or a chatty friend turns quiet and stares off.

Clear fluid or blood dripping from the nose or ears, one pupil bigger than the other, or passing out and not waking up mean call 911 at once.