Knock on Wood

I found this information at http://members.aol.com/MorelandC/HaveOriginsData.htm

Meaning: If good luck is willing. Example: I am sure that your tax returns will not be audited, knock on wood. How funny that this was the context that inspired the post!
Origin: One theory is that it originated in the middle ages when there were in circulation, pieces of the Holy Rood or Cross on which Jesus was crucified. To touch one of these was supposed to bring good luck hence touch wood for good luck. There were also several Holy Foreskins around at the time ..but that was a different story! Thanks to Zebulun
Alternatively: The Druids, who worshipped trees, especially Oaks, wore a piece of Oak around their neck to ward off evil spirits. Hence touch wood for good luck. Thanks to Cynthia Blackledge
Alternatively: You knock on wood because of those wily Wood Sprites. In medieval times, people believed in mischievous creatures known as sprites. Sprites are actually spirits or ghosts who were reputed to enjoy causing trouble and wreaking havoc in the lives of the living. Among the most mischievous were wood sprites. If you were to mention something good, the wood sprites would try to foul it up. The thought was that if you knocked on the wood when you said these things, the wood sprites would not be able to hear you because of the knocking sound. Hence they would leave you alone. Thanks to Coach Alan and Fluke
Anyone else out there have information?