(Cross-posted at Jennifer's History and Stuff.)
If you say something to tempt fate, knocking on wood will protect you. (Example: "I'm the only one in my house to not get the flu. Knock wood.")
This superstition can be traced to the early 1800s, with some people believing it invokes the wood of the crucifix for protection.
The real origin seems to be a children’s game from the early 1800s called “Tig-touch-wood”. Children playing tag were “safe” when they touched wood. Touching wood for luck became knocking on wood for luck in the U.S. and Germany.
Reference: Most of the material from this post was found in Steve Roud's The Penguin guide to the superstitions of Britain and Ireland.