Snape frowned uncertainly when Al asked him if he'd ever tried writing on skin before; he didn't think now was the time for him to expound on the theory behind Blood Quills, or the uses to which they could be put.
But Al's innocent enthusiasm was impossible to resist. He gave a lopsided grin and held out his hand. Indeed, the tips of his fingers were dark with stains - ink, and some of the more stubborn potion ingredients.
"My hand is yours," he declared wryly, sending himself up.