Knock wood

 The question of completely cleaning one's plate is to me similar to the one of "knocking wood" when one makes a mild boast.  I explained this custom to a Cambodian once as a superstition that if the spirits hear one boasting they may decide to put you down otherwise.  Of course it is mainly to tell others that you are aware this is a boast.

Yes — and I think the resemblance is deeper than just “both are superstitions.”

“Knock on wood” is almost a little piece of socially understood ritual language. When you say something favorable about yourself — “I haven't been sick all year,” “the roof has never leaked,” whatever — the gesture says, in effect, I realize I have just tempted fate, and I am not claiming immunity from it. It modifies the boast.

There is an interesting ambiguity in the custom you described to your Cambodian acquaintance. You gave the folk-causal explanation: spirits may hear the boast and punish you. But in ordinary use, the ritual doesn't require the speaker actually to believe that mechanism. It works perfectly well as a cultural signal: “I'm making a small boast, and I know I'm making one.”

That makes it rather like manners. The underlying metaphysics can evaporate while the social meaning survives.

And I think your plate-cleaning habit has a similar structure, though with an opposite moral direction. “Clean your plate” can begin as a practical injunction — don't waste food — but it can acquire a ritual significance quite beyond the practical one. Eventually the last little green fleck isn't really about nutrition or thrift anymore. It is about having fulfilled an obligation properly. Leaving it there feels somehow wrong, even when no consequential harm follows.

There's a lovely anthropological idea lurking here: many customs are compressed explanations. Somebody long ago attached a concrete reason to a behavior, and generations later the reason may become uncertain or even absurd while the behavior remains because it has acquired emotional and social force.

Your “the spirits may hear me” explanation for knocking wood is consequently rather elegant: it explains the old-fashioned version of the custom, while your “I'm signaling that I know this is a boast” explanation describes what the custom may actually be doing in modern conversation.

And, intriguingly, both customs contain a little bit of humility. “I am not entitled to assume that good fortune will continue” and “I am not entitled to waste what has been given to me.” That's more interesting than mere superstition.

Sometimes it's "knock on wood," sometimes just "knock wood."

Exactly. “Knock wood” is the wonderfully stripped-down version. The on disappears, rather as though everyone already knows the ritual and the preposition is expendable.

There is also an interesting difference in feel. “Knock on wood” sounds like an actual instruction describing the physical act. “Knock wood” sounds more like a formula—almost an interjection meaning “let me perform the customary anti-jinx maneuver.”

And of course one can get the very compressed version:

    “We've had no trouble with the car this year—knock wood.”

At that point nobody is literally commanding anyone to knock anything. The phrase has become a little grammaticalized piece of conversational magic.

English is full of these fossils: expressions whose original literal machinery has partly disappeared while the social function remains.
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