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. Thinking Putty® helps build hand and finger strength through a fabulous tactile play experience with unique, unexpected properties and provides relaxing, yet stimulating interaction for anyone with sensory integration issues. Crazy Aaron's putty is non-toxic, won't dry out, and won't leave a sticky or slippery residue on your fingers, so it's great for stretching, molding, bouncing, tearing, popping and playing by kids and adults alike.