Decoder pen

The decoder pen, yes-no pen, yes-know pen or magic pen book is a combination of decoder pen or marker specially designed to reveal encoded pictures or writing, in the form of answers to questions or hidden parts of pictures, with specially created children's books with hidden words and pictures. They were most popular in the 1970s, but continue to be sold at gift shops. Often they were in the form of trivia games, which enabled the player to guess before revealing the answer.