Good points, Severn, about what can be steps in the becoming of folk. (This is a little like The Velveteen Rabbit, or How Toys Become Real, isn't it?) My kids got a fairly eclectic bunch of lullabyes too, The Great Silkie of Shule Skerrie, Brid Og Ni Maille, anything my daughter described as a "long, calm song." I agree absolutely that a song being around long enough that people learn it from other people singing (or better yet, feel they have "always" known it), and that people have forgotten who wrote it, can certainly signal that a song has become "folk" (much looser definition than "traditional") but there are surely other paths.