The blow is hard enough to make the brain hit against the skull. The brain can swell, a bruise can occur on the brain's surface, or bleeding can occur in or around the brain. The blow to the head is usually sudden and forceful.
Examples are a fall to the ground, a punch to the head, or a car accident.
Most people with a concussion recover completely. Repeated concussions may lead to damage that may last for a long time. Some children have no immediate symptoms, but grow sluggish and moody within a few hours. Some of the symptoms may persist for several weeks. Treatment can ease some symptoms, but the concussion must heal on its own.