Virtually all tribal societies transmit tradition through the medium of songs. In this Season of the Ancestors, we remember the Mothers and Fathers by gathering to tell their stories and sing their songs.
Green Grow the Rushes-O has been part of the Younger Witchery since the early 1960s at very least (some would say for much longer than that); Robert Cochrane, father of the modern Old Craft movement, mentions the song in his letters. This version, which I learned from Feri elder Alison Harlow in 1980, functions both as riddle-song and teaching-song. (Any teacher can tell you that making the student figure things out for herself is the most effective heuristic.)
The mysterious Eleven Words of Power are said in witch legend to be, in fact, the very basis of all existence; but of this I should probably say no more.