Knock on Wood.
Knocking on Wood is one of history's most enduring superstitions. It comes from thousands of years of mythology, folklore, and religious beliefs that trees are sacred, and believed that trees housed gods and nature spirits. People have laid hands on the trunks of a trees to seek favors from tree gods and nature spirits, and have believed that knocking on wood would wake up and shake out mischievous tree sprites and fairies.
I think these little tree sprites would happily stay napping if it wasn't for that pesky woodpecker!
If you're interested in my working process, I've posted a summary here.