Even a relatively minor blow to the head can lead to serious trouble if it causes bleeding in or around the brain. The brain is a soft organ. It is covered with blood vessels that can be damaged when the brain moves rapidly within the skull as a result of a severe blow to the head, such as when a person's head strikes the pavement after falling off a moving bicycle. During the fall, the body, including the brain, moves rapidly through the air. When the head strikes the pavement, the skull stops suddenly, but the brain keeps moving rapidly until it strikes the inside of the skull. The resulting forces can rupture blood vessels or fracture the skull. When blood collects in the small space between the brain and the skull, it presses against the brain and pushes it against the skull. The resulting pressure can severely damage the brain unless a neurosurgeon removes the blood. Fortunately, blood is easy to detect with a computed tomography (CT) scan. When emergency department doctors see patients with head injuries, even a minor bump on the head, they usually order a CT scan of the head to be sure that blood is not present. Methods that use a patient's history and examination can identify patients who have enough damage to cause bleeding. Clinical prediction rules can identify patients who have a head injury that is bad enough to cause them to lose consciousness. Many patients do not lose consciousness after a head injury, and these rules might not be safe to use in them.