In many cultures, it’s a common superstition for people to knock their knuckles on a piece of wood to bring themselves good fortune or ward off bad luck.

Yet while the phrase “knock on wood”—or “touch wood” in Britain—has been part of the vernacular since at the least the 19th century, there seems to be little agreement on how it originated.

One common explanation traces the phenomenon to ancient pagan cultures such as the Celts, who believed that spirits and gods resided in trees. Knocking on tree trunks may have served to rouse the spirits and call on their protection, but it could have also been a way of showing gratitude for a stroke of good luck.

Yet another theory is that people knocked on wood to chase away evil spirits or prevent them from listening in when they boasted about their luck, thereby preventing a reversal of fortune. Christians, meanwhile, have often linked the practice to the wood of the cross from Christ’s crucifixion.

Other researchers consider knocking on wood a more recent phenomenon. In his book The Lore of the Playground, British folklorist Steve Roud traces the practice to a 19th century children’s game called “Tiggy Touchwood,” a type of tag in which players were immune from being caught whenever they touched a piece of wood such as a door or a tree.

“Given that the game was concerned with ‘protection,’ and was well known to adults as well as children, it is almost certainly the origin of our modern superstitious practice of saying, ‘Touch wood,’” he argues. “The claim that the latter goes back to when we believed in tree spirits is complete nonsense.”

While the origins of “knock on wood” may never be known for certain, the superstition remains popular around the globe and has even given rise to several local variations. Turkish people often pull on one earlobe and knock on wood twice to ward off a jinx. Italians, meanwhile, say the phrase “touch iron” when trying to avoid tempting fate.