Uh that should be Motley Crue -

I would give up now, but this whole knock on wood thing has got me speculating.

Basically the phrase in various incarnations is popular all over the globe.

I find it fascinating that we knock on doors now. Do you suppose that prior to there being doors of wood upon which to announce arrival, get attention, gain entry, that human kind rapped on wigwam, yurt, tepee, etc.? Likely not, as more often than not these dwellings were habituated by more than one family. All belonged to all and so any could just enter. Perhaps they hailed vocally from out of "flap" if not too closely linked? Wonder what the protocol was.

So where did the custom of knocking on the door come from? I like to think as social and nomadic living gave way to individual permanent housing, that with the introduction of wooden doors, the tradition of knocking on wood came as a kind of blessing upon entering the home of your fellow.   

I like to think sometimes, so long as it is not too taxing.