The LeapPad series is a family of interactive learning devices for kids of varying age groups. To start, place any book from the Leapfrog Library inside the unit, insert its cartridge, and point the stylus at any graphic or word in the book to hear its associated sound. The stylus receives low-power radio frequencies from a grid under the plastic shell and book. The audio sounds, games and interactive activities enhance the learning process and students can select from books covering reading fluency, vocabulary, and comprehension, math, science, social studies, and grammar. Over one million units have been sold so far and it was named the number one selling toy in the Electronic Learning Aid category, according to NPD Funworld 2004 data.