Traditionally, when you speak of your own good fortune, you follow up with a quick knock on a piece of wood to keep your luck from going bad. More recently, simply saying the phrase "knock on wood"—or "touch wood" in the UK—has replaced literal knocking. Where'd all this come from?
Before modern religions came around to spoil the party with their rules about idolatry, many pagan groups and other cultures—from Ireland to India to elsewhere in the world—worshipped or mythologized trees. Some peoples used trees as oracles. Some incorporated them into worship rituals. And some, like the ancient Celts, regarded them as the homes of certain spirits and gods.
The first possible origin of knocking on wood is that it's a more modern equivalent to the ruckus that pagan Europeans raised to chase evil spirits away from their homes or to prevent them from hearing about, and thereby ruin, a person's good luck.
The other suggested origin is that some of these tree worshippers laid their hands on a tree when asking for favor from the spirits or gods who lived inside it, or that they would touch and thank the tree after a run of good luck as a show of gratitude to the supernatural powers. Over the centuries, the religious rite may have morphed into the superstitious knock that acknowledges luck and keeps it going.
"In either case, you are seeking protection against envy and anger," Bechtel wrote. "The envy of evil spirits and the anger of the gods, who take a dim view on mortals bearing too much pride, and who get especially annoyed when they're responsible for your run of good luck and you're not grateful."
Another possibility? That it simply came from a Victorian-era children's game called Tig Touch-Wood. As described in the 1891 book The Boy's Modern Playmate, "Tig" is the person who is "It," and after a number of trees have been chosen as bases, "as long as the player is touching one of these authorized posts, Tig cannot touch him; his only chance is to catch him while flitting from one post to another."
But for anyone who may be superstitious, we're sure knocking on wood is no child's play.
This piece was updated in March 2019.