Answer: Flexi Discs

Throughout the 20th century, music was widely distributed on very inexpensive records known as flexi discs. The original flexi discs were very thin vinyl (so thin, in fact, they often had to be weighted down with a few pennies to keep them flat on the turntable) and were used to promote brands and musical acts very inexpensively. The Beatles, for example, would mail out Christmas cards to members of their fan club that included greetings from the band and music all packaged on a little flexible disc included with the card.

Over time, manufacturers refined the technique and were able to drive costs down even further. The biggest innovation in terms of cost cutting and further market reach was the introduction of cardboard flexi discs. The surface of the cardboard was coated with a very thin layer of plastic laminate etched with a record groove. Between the 1950s and 1980s, millions of these cardboard flexi discs were distributed as part of cereal boxes. Cut the back of the box off, poke a hole in the center of your cutout with a pencil, pop it on a turntable, and suddenly your Wheaties box was singing.