“Knock on Wood” is the platonic ideal of a Stax soul single. It’s got a beat you can dance to. Its horns are towering, shiny and perfect. It’s an ode to a lover that was conceived during a thunderstorm while Floyd and Cropper were holed up at the Lorraine Motel, an interracial songwriting duo writing at maybe the most notorious hotel in American history (it’s where Martin Luther King Jr. was assassinated). It’s got music history in its backbeat, a No. 1 hit that, seeing Floyd perform 50 years after it was written, melts away the years and bridges the gap in decades.