Doctors call an injury a trauma. This is a sudden and unexpected event that puts more stress on a part of the body than it can put up with. As a result, the skin is bruised or cut, ligaments might be torn, bones can be broken and so on.

Twisting your ankle because you have been running over uneven ground is a good example. The body tells you that it needs to repair itself by sending out pain signals. At the same time, if there is any bleeding, special plate cells go to the place of bleeding and solidify as soon as they reach the air, causing a blood clot. That is all quite normal.

Some traumas can be reduced. For example, by having well-made trainers and by having automobiles fitted with crumple zones and airbags. But all the same, it is unlikely you will get through life without an injury of some kind. It's good to know that the body can mostly heal itself, and it does not usually need a trip to hospital to find that out.