In my house, ink on skin happens when my teenage kids want to make a "mental" note. With a pen, or sometimes, in a moment of hormonal lunacy, a Sharpie. Perhaps you have a creative toddler in your house, or perhaps you even do it yourself. I've been known to jot down a critically important "must remember" note on the nearest available surface -- even if it is the palm of my right hand!
But once you don't need that note any more, you're stuck with it: ink just doesn't come out so easily. It's one thing to scour your own arm raw trying to remove that black stain, but just try doing that on an outraged toddler! It's a three-man job... unless, that is, you have a teabag.