First published on my blog at evenlodesfriend dot com. Inspired by my husband, who has a similar 'shark bite'.

John first noticed it when Spring came, and Sherlock started swanning around the house in his sheet. A ragged scallop of raised keloid tissue that scooped around Sherlock's right elbow.

'Shark bite,' Sherlock said, with surprising nonchalance.

John examined it. The scarring was deep, and he could even see the indentations of those cruel razor teeth.

'Holiday in South Africa when I was in my teens. I was swimming, and the next thing I knew this shark had my arm in its mouth. I punched it on the nose, and it let go.' He shrugged, as if it were an everyday occurrence, and then regaled John with details of the beast's species, hunting habits and vulnerabilities – namely the aforesaid sensitive nose.

John found out the truth a few months later, during one of Mycroft's little episodes of abduction. The subject of Sherlock's sojourn in South Africa came up, and Mycroft laughed.

'Oh, God, he's been telling that stupid shark story again! Sherlock's never been to South Africa!'

John scowled, but put his head on one side, curious about the truth.

'It was one summer when he was about twelve. He was told not to go playing in one of the woods on the estate because of the forestry going on there. Felling trees and so on - it was dangerous. But being Sherlock, he went anyway. Typically, he didn't get crushed by a falling tree. He fell off his bike, and got his arm tangled up in some barbed wire in a hedge instead. Cut it to ribbons, terrible mess.

'Father was absolutely furious, wouldn't even look at him because he had disobeyed, insisted it was just a graze and Sherlock was making a ridiculous fuss over nothing. Wouldn't take him to have it stitched either, and Mummy couldn't drive, so she just had to bandage it up as best she could. Of course, it healed badly, and left that dreadful scar.

'When he went back to school after the vac, the other boys saw it and wanted to know how he got it. So he made up that silly story about the shark. It got him quite a lot of attention to begin with, but of course, being Sherlock, he overplayed his hand, and got too full of himself, and that was the end of his brief acclaim.'

Afterwards, John thought for a long time. He thought about the barbed wire. He thought about the spotted marks around the fat ribbon of raised, whitened flesh that looked like the marks from sharks teeth. He was a doctor, and he knew about scarring and healing, and what kind of depth of wound would leave a mess like that. He thought about the boys at the school. And then he thought about the kind of father who, out of pique, would neglect the proper attention that a wound like that would require.

John never mentioned Sherlock's fantasy visit to South Africa again to anyone. But he always referred to Sherlock's scar as a shark bite. Because in his opinion, that was exactly what it was.