In the family gift exchange I got my 13 year old brother, who is really into spy stuff. In thinking of what I could make him I came across an instructable for a hollowed out book. I had done it before and it worked really well, so the first inspiration for this gift was: http://www.instructables.com/id/secret-book/ In addition to the book I thought it would be cool to put something inside that was spy related: fake passports, finger print dust, etc. I settled on some kind of invisible ink and a writing utensil. After some investigation, some trial and error, and some inspiration from: http://www.instructables.com/id/25-Cent-Invisible-InkPen-UV-light-visible/ The following is the steps it took to build a spy hollowed book, with a ordinary (in appearance) highlighter, and a black light to reveal the secret information.