The blacklight spy pen is really handy! Write secret messages using the included invisible ink, which only become visible under black light! Flip the pen over and use the blacklight end to check for counterfeit money, lint on your clothing, or anywhere you want to illuminate blacklight reactive items. Makes white cotton shirts glow like crazy! The light also makes a great flashlight for reading in a dark club. Comes with 2 ink refills and an extra set of 3 replaceable AG3 batteries.