A "secret decoder ring" was an inexpensive toy popular among young children during the first half of the 20th Century. It was often included as a "toy prize" in boxes of breakfast cereal and snack foods, such as Cracker Jack. Among the most famous decoder rings were the ones given to kids by the makers of Ovaltine.
Decoder rings were a fun way for kids to tap into the fascination that children often have with encryption, ciphers, and "secret codes" that they could use to send hidden messages back and forth to one another. A number of companies that marketed the rings to children (such as Ovaltine) often included "secret messages" on various radio shows aimed at children. The kids needed to use their decoder rings to decode the "secret message" (which was usually a "fun" phrase or a commercial pitch to buy their product).
