Teenager eyed my calf the other day and suddenly got all excited - "Hey, is that a really cool tattoo on your leg".
It was a grease mark from where I'd leaned up against the front cog, which left an arc of black teeth up my calf. It was the Mark of Chain. Or perhaps the Mark of Cog. It's certainly the mark of a filthy bike that needs a wash and degrease.
Teenager wandered off all disappointed. I went to the bathroom where it took a good five minutes of scrubbing with a stiff brush to remove it.