Knock Knock Who's There was written by John Carter and Geoff Stephens and performed by Welsh singer Mary Hopkin as the United Kingdom entry at the Eurovision Song Contest in 1970, where it finished second. Released as a single, it became a UK hit and remained associated with Hopkin's early career. Decades later the short sung title line was clipped for soundboards and short videos, where its familiar joke setup made it easy to reuse for entrances, reveals and playful call and response edits.