Why do some people need pain killers just to sit in a dentist's chair? And how is it possible for rugby players to stay in the game even with a broken arm?
Sydney University lecturer Lorimer Moseley has spent a lot of time studying these questions - and come up with some not-so-surprising answers.
"The brain is capable of many wonderful things, based on its perception of how the body is doing and the risks to which the body seems to be exposed," he said. "The more we learn about pain, the more it becomes apparent that tissue damage is neither necessary nor sufficient to cause it."
In a recent research project, published in the latest edition of the journal Current Biology, Moseley and his collaborators found that the size of an aching limb can affect the perception of pain.
Ten people with aching arms looked at their damaged limbs either through glass that magnified or minimised. If the limb was made to look large, the patient's perception of pain was amplified. When the swelling was artificially reduced, the real swelling reduced and the patient felt less pain.
The explanation seemed to be that the brain was responding to inputs and acting accordingly.
So, when you next bump your head, don't look in the mirror and marvel at the size of the swelling. A less painful response would be to imagine no swelling there at all.