why do we become unconsious when hit on head

Different part of your brain weighs differently, so when you hit your head, the movement of different region of the brain is different, some region moves faster while some move slower, causing damage to your brain tissue. This effect is called a Traumatic Brain Injury, TBI in short. It ranges over three level, mild moderate or severe. The effects ranges from a slight dizziness, to being unconscious. TBI can lead to Vegetative State, Unconscious ( if serious enough would result in a coma), or disability. The damage to the brain tissue may result in the irregular signals send by the nerves. But up till now, the effects and the direct causes of consciousness and unconsciousness have not yet been discovered. Only theories which are yet to be proven exists. The brain is an amazing organ which is still far from being fully understood.