I’d always thought it was meant to ward off the devil or something by knocking on wood, but I’d learned late in life that I’d been doing it completely wrong. You weren’t supposed to knock on wood, but on anything but wood. When you said something unlucky, any listening evil faeries would work to make it happen, but if you followed it up by saying “knock on wood,” and instead knocked on some plastic, they’d think you were just talking nonsense.
