Why do we... knock on wood?

In the world of Rome's Revolution, Rei is always saying something that Rome has no clue what he means. Many of our colloquial expressions and actions require a cultural context to make any sense at all.

Today's question is: Why do we knock on wood for luck?

A long time ago, when people were very superstitious and believe in elves, fairies and woodland spirits, the spirits that lived in trees were considered to be kind and generous. They had a very positive vibe about them. You could touch a tree, say something nice to the spirit within and if you were lucky, the spirit would help you with your wish.

However, once the tree was cut down, the spirit's vitality was reduced. As the wood was sliced into boards, the spirit's ability to even stay awake was severely impaired.

So when you knocked on wood, you presumably woke up what remained of the spirit from within the tree long enough to hear your request and invocation and to the extent that they might grant your wish, they might do so.

Tomorrow, why do we shake hands?