Knocking on wood is a superstitious tradition of literally touching, tapping, or knocking on wood, or merely stating that one is doing or intending to do so, in order to avoid "tempting fate" after making a favorable prediction or boast, or after experiencing good luck. The phrase is often used to ward off bad luck or to express gratitude for good fortune. The origin of the phrase is uncertain, but many believe it originated with pagan groups and cultures around the world, such as the Celts, who worshipped and mythologized trees. These groups believed the trees were home to their various gods and with the touch of wood, a spirit could bring protection. It may also have served as a way to show gratitude that would bring good fortune. In modern times, the phrase is used more as a figure of speech, and people may knock on any nearby surface, not just wood.