The Detecta Mark Anti-Theft Set includes 2 pens that write with permanent, invisible ink which can only be seen under ultraviolet (UV) light and a stylish UV pen light. With this set you can mark your valuables with the UV pen that police can use for identification if stolen and recovered. The Detecta Mark Anti-Theft Set can also be used to check for counterfeit money or as a spy toy for children to write secret messages. The UV flashlight measures 5 inches long and requires 3 LR-41 batteries (included, along with an extra set of 3).