Often used to encode top-secret information, invisible ink has a long and interesting history dating back to 4th century BC. There are all kinds of ways to make and read invisible ink (like heat and lemon juice), but our Invisible Ink Thinking Putty uses ultraviolet light to mimic blue ink on a sheet of green paper. Write with the Glow Charger and watch with wonder as your words evaporate into thin air. For ages 8 and up. WARNING: CHOKING HAZARD - Small Parts. Not for children under 3yrs.