"Why do people knock on wood?" While all of us here on the Newton Falls Public Library staff are familiar with the expression "knock on wood," typically used after remarking on one's own good fortune, none of us were sure where it originated.
According to A Dictionary of Superstitions, edited by Iona Opie and Moira Tatem, "knock on wood" (or "touch wood," which is the British equivalent of the expression) has been around for a long time. In nineteenth-century England, it was already considered an old superstition.
While there's no hard-and-fast explanation for how the custom came about, Peter Lorie gives a few theories in his book, Superstitions. It may have something to do with the cross of Jesus being made from wood, or it may date back to the ancient Celts who viewed trees as having spiritual significance. According to Lorie, touching wood grounds evil spirits and renders them harmless.
Matt Soniak in his Mental Floss article "Why Do We Knock on Wood?" also traces the superstition back to the Celts. They may have touched trees when asking for a favor or to show gratitude to the spirit inside it. The custom eventually morphed to touching wood after mentioning a streak of good luck. It may also have originated from the belief that loud noises scare away evil spirits, so the sound of knocking is meant to frighten away any lurking nearby.