In the superstition known as knocking on wood, people traditionally touch or knock on wood to prevent bad luck.
The custom is especially common in English-speaking cultures. Someone may mention good fortune, then knock on a wooden surface to avoid tempting fate or to protect the good outcome from being reversed. In everyday speech, people may say “touch wood” instead.
The origin is uncertain. Explanations have connected the practice with pre-Christian beliefs about trees and spirits, as well as with Christian-era associations involving sacred wood. Historians generally caution that no single origin has been proven conclusively.
The action is not a scientific safeguard and does not have one universal form. Some people knock three times, while others simply touch a nearby wooden object. The important feature is the material: the custom specifically involves wood, which distinguishes it from other protective gestures such as crossing the fingers or throwing spilled salt over a shoulder.