Knock wood

I never learned this. I'd heard that it had to do with invoking spirits of the wood, and dated back to when dryads inhabited trees. But something about that doesn't ring right. Maybe a Greek woodcutter would do some ceremonies before cutting down a tree. The connection to modern knocking on wood seems tenuous. After all, most of the wood went into fires, and I've never heard of anybody "knocking" firewood before tossing it in the stove.

According to the font of occasional knowledge sometimes this is to ward off some negative statement, and sometimes (Vietnam) to invoke something positive. Most of the examples are from Euro-MidEast culture--which if the tradition is old enough we can consider as a single thing.

Wikipedia cites as "unreliable" an Italian tradition of "touching iron" instead, though it's no hardship to find other sources testifying to it. I had a vague notion about how that might tie in with "touch wood", but the alternatives for touching that the article cites don't quite fit the notion.

Some rituals are complex (pull earlobe and knock twice), but most are perfectly simple.

Some of the ritual's meaning probably comes from the senses--how does it feel to touch wood after hearing something you fear? It's a contrast with the words--you feel something solid and different. It takes your attention away from the words of fear. Knocking, if not touching, makes noise too--a counter-sound?. It feels like you are doing something for yourself and the others, even if only symbolically, against the fearsome words.

Maybe consistency is a bridge too far in superstitions. Still, why not use stone? Reasons for: Stone is solid and strong, in contrast to our fuzzy words and flimsy hopes. Stones could be sacred too. Reasons against: Chthonic spirits seem more alien, maybe less friendly, than trees. It makes little noise. It probaby hurts more to knock stone.