So the new Retro 51 glow-in-the-dark pen has got me thinking about glow-in-the-dark ink. I haven't seen any. We already have some invisible ink, but that's different. Glow-in-the-dark items that I own (novelty clothes, watch markings) are visible in daylight but glow when it's dark; quite different from invisible except under UV light. I thought if such an ink existed, it would make a fun Halloween gift. Does anyone know of one?