There is a certain type of person (usually creative and slightly absent-minded) that needs to keep a pen on them at all times, so sticks one in their hair. I had an art teacher once who would get to the end of the day and find 5 or 6 pens that she’d forgotten about.
Obviously, you need longer hair to do this. You also need to remember one thing in particular: putting the lid on is a sensible idea. Otherwise, you’ll end up with an interesting, if slightly abstract tattoo. As well as a lot of strange looks on the subway.