When you knock on wood after mentioning good fortune so the evil spirits will not ruin things for you.
The American version is "knock on wood", while the British version is merely
"touch wood". The tradition traces back to an ancient pagan belief that spirits
resided in trees, particularly Oaks, and that by knocking on or touching the wood,
you were paying a small tribute to them by remembering or acknowledging them,
and could call on them for protection against ill-fortune. With the second knock, you were thanking them for their continued blessings and good luck.
It may be traced back even further to an ancient Greek belief, according to Sauren
Dessai "that if they touched an Oak tree, they communicate with Zeus, who
would protect them from misfortune."
42 Days Until Christmas....Happy Thursday!