Anything written by this Secret Spy Message Invisible Ink Pen what can only be seen under ultraviolet light, allowing for fun games between friends and messages that are only for specific recipients. Also fun for kids of all ages. You can write invisible messages and read by turning on the mini UV light on the top of pen. Kids will love these spy pens. Kids love using this kind of pen to keep their secret and play detective spy games. Permanent but washable!