Valentine's Day Secret Sweethearts Invisible Ink Pen with UV Light "With the Secret Sweethearts Pen you can continue to write those sappy love letters without the fear of someone else reading them. Each 5" (12.7 cm) plastic pen features ink that is made of special high-brightness compounds that are invisible to the naked eye, but magically illuminate when exposed to ultraviolet light. A black light is conveniently located on the opposite end of the pen, allowing you to read any secret notes sent to you by your lover. Battery included."