Brains being our centres of personality, vision, hearing, motor control, consciousness and, in short, everything that makes us functioning and human, there is a reason why they are encased in very hard bone. And why it is reasonable to assume that breaching that bone is catastrophic.

But not always. Margret Wegner discovered this when, aged four, she tripped and fell onto a pencil she was holding - and it passed through her cheek into her brain. Last weekend, 55 years and the odd headache later, she had it removed by surgeons in Berlin. Her experience isn't unprecedented; earlier this year a 77-year-old Chinese woman, Jin Guangying, had a rusty bullet removed from her head. It had been there ever since a Japanese patrol had fired at her when she was 13.

A couple of years ago, Leonard Woronowicz, a retired Polish teacher, popped round to the hospital to get some headache pills. He had fallen in his kitchen a few days earlier, so doctors x-rayed him to see whether he had cracked his skull. They found a 5in blade within it. "There was a small gash on the side of my head but I thought it would soon heal so I put a plaster on it and left it," said Woronowicz later. "I didn't even guess what had happened when the next day I wanted to cut a piece of bread but couldn't find the kitchen knife." You see his point. "Where's that pesky knife?" he wouldn't have thought. "Oh, I know. In my head."

Are there empty areas of the brain in which foreign objects might find a berth? "No, I don't think you can say that," says Keyoumars Ashkan, a consultant neurosurgeon at King's College Hospital. "There are areas that we don't know the function of very well as yet." What is true is that the brain can "feel for the rest of the body, but when it comes to itself, it has no sensation of pain. That's why, for, example, I do a lot of awake operations. Once I'm inside the patient's brain I can operate without causing pain to them." With Parkinson's patients, for example, he can see whether he has stilled the tremors in their legs, rather than their hands. And why, once the blade was lodged in Woronowicz's skull, he didn't know it was there.

As for those who have spent decades harbouring alien objects, "there are lots of areas of the brain which are critical for everyday function, those that make you see, walk, talk, breathe, etc," says Ashkan. But "there are also areas that are less eloquent, so the function is less clearly defined. If the injury was to those areas, the injury may well be compatible with a near-normal life." So while detailed tests might show up slight changes in cognition, mood, personality or memory - "what makes you you" - these are hard to define. Jin Guangying's relatives reported periodic foamings at the mouth and talkings of nonsense.

The moral? Well, no running with pencils, perhaps. Or knives. Or nail guns. As for dodging bullets - hmmm. Depending on where you live, that might be slightly more difficult to arrange.