Tap on a wooden surface for gook luck or to keep
from putting a jinx on yourself for having mentioned some hope or dream aloud.
If good luck is willing. I am sure that your tax returns will not be audited, knock on wood. 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.