May Somebody Melt Some Lead Into Satan's Ear

In my household, we're very superstitious about knocking on wood. It's not so surprising in my case, since I'm kind of a big flake, but you wouldn't expect someone like Guy to be superstitious; he's otherwise so coldly logical.

However, once I did a little research, it all made a lot more sense. When Guy utters something that seems to be tempting fate--then looks wildly around the room for a piece of wood to knock on, unable to rest until he's done so--he's not really being illogical at all. He's simply letting the wood spirits know he is there. That's just plain good sense:

"The phrase is used by people who rap their knuckles on a piece of wood hoping to stave off bad luck. In the UK the phrase 'touch wood is used - often jokingly by tapping one's head. The derivation may be the association that wood and trees have with good spirits in mythology, or with the Christian cross. It used to be considered good luck to tap trees to let the wood spirits within know you were there. Traditions of this sort still persist in Ireland."

Wikipedia offers this comprehensive tour of knocking on wood around the world:

In Belgium (Flanders), they say: "hout vasthouden", which would translate as "touching or holding wood". If possible when saying this, they touch a table or door or something else made of wood.

In Denmark, "bank under bordet" (knock under the table) is a commonly used phrase, which is often used as a part of the phrase "7-9-13, bank under bordet", where "7-9-13" is just another way to say touch wood.

In Germany, the version "auf Holz klopfen" (knock on wood) can be accompanied by the phrase "Toi, toi, toi" (probably derived from the Old German word for 'Devil' or from other traditions of spitting over your left shoulder three times for the purpose of warding off the Devil) which is still used as a charm to ward off evil or as a good luck charm for thespians out of superstition that wishing an actor good luck brings the opposite.

In France, "toucher du bois" is used with the exact same meaning.

In Italy, a similar superstition exists, it's said "Toccare ferro" and the meaning is similar: one must touch metal, preferably iron.

In Netherlands, the term "afkloppen" (knock off), is used, sometimes accompanied by "op ongeverfd hout" (on unpainted wood).

In Norway, the term "bank i bordet" ("knock the table" or rather: "knock the wooden board" ('bordet' is an ambiguity)), is used. In Norway, it is also sometimes used to stress that you're telling the truth (akin to saying "I swear to god that...").

In Poland, the version of this charm is "odpukać w niemalowane" [knocking on unpainted (wood)]. As the name of the charm suggests, the charm only works if one knocks on unpainted wood.

In Portugal, the version, which has a similar meaning to the others all around the world, is called "bater na madeira", and when someone does this, "lagarto, lagarto, lagarto" (lizard) is uttered.

In Switzerland, the Swiss German version is "Holz alange" (touch wood) – but while saying it, knocking on or tapping wood is still required. A simple touch is not enough.

In Sweden, the phrase "ta i trä" (touch wood) is commonly used as a part of the phrase "peppar peppar, ta i trä" (pepper pepper, touch wood), the double "pepper" also being used to ward off a temptation of fate. It's often shortened to just saying "peppar peppar" while knocking on wood.

In Turkey, "tahtaya vur" (knock on wood) is used. Usually, someone else will answer: "Şeytan kulağına kurşun" (May somebody melt some lead into Satan's ear).

In India, it's said as "Nazar Na Lage" (let there be no evil eye), and in Hindi, the meaning is similar. It is used when something seems too good, like saying "touch wood"; it's said as "Kannu pada Pooguthu" (let there be no evil eye), in Tamil.

In United States, It's predated by a much more popular and more meaningful saying "Stomp on Wood" which was used in the early 1800s. It was said to originate from early settlers when they would stomp the floor in their log cabins to avoid bad luck. The saying "Knocking on Wood" was recorded only after the early 1900s.