Ultraviolet Light Pen.
*** With this write and light UV glow pen, you can write notes and messages in secret. The pen uses an "Invisible Ink" which can only be seen under UV light. Simply write your message with the special pen, and when you need to read it, shine the small UV light onto the note and instantly your message appears. Ideal for leaving secret messages on post-it notes. Try it on colour paper to for best results. The pen light can also be used to reveal counterfeit money or act as an emergency flashlight.