Question: Where did the superstition knock on wood come from?

Disambiguated questions and answers:
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question: Where did the superstition knock on wood because of protection from supernatural beings come from?
short_answers: Celtic or German folklore

context: The origin of the custom may be in German folklore, wherein supernatural beings are thought to live in trees, and can be invoked for protection. One explanation states that the tradition derived from the Pagans who thought that trees were the homes of fairies, spirits, dryads and many other mystical creatures. In these instances, people might knock on or touch wood to request good luck, or to distract spirits with evil intentions. When in need of a favour or some good luck, one politely mentioned this wish to a tree and then touched the bark, representing the first "knock". The second "knock" was to say "thank you". The knocking was also supposed to prevent evil spirits from hearing your speech and as such stop them from interfering. Alternatively, some traditions have it that by knocking upon wood, you would awaken and release the benevolent wood fairies that dwelt there.
question: Where did the superstition knock on wood to distract spirits come from?
short_answers: Pagans
wikipage: Knocking on wood

context: No context provided
question: From what object did the superstition knock on wood come from?
short_answers: wooden crucifixes

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question: What is the more modern theory for where the superstition knock on wood come from?
short_answers: a form of tag called "Tiggy Touchwood"

Long answers:
knowledge: content: Knocking on wood, also touching wood (the usual phrase and action for speakers of English), is an apotropaic tradition of literally touching, tapping, or knocking on wood, or merely stating that one is doing or intending to do so, in order to avoid "tempting fate" after making a favorable prediction or boast, or a declaration concerning one's own death or another unfavorable situation.
wikipage: Knocking on wood

content: Another explanation links the practice to wooden crucifixes.[1] A more modern theory from folklore researcher Steve Roud suggests it derives from a form of tag called "Tiggy Touchwood" in which players are safe from being tagged if they are touching wood.[1]
wikipage: Knocking on wood Origin
long_answer: Knocking on wood is a tradition of literally touching, tapping, or knocking on wood, or merely stating that one is doing or intending to do so, in order to avoid "tempting fate" after making a favorable prediction or boast, or a declaration concerning one's own death or another unfavorable situation. The origin of the custom may be in Celtic or German folklore, wherein supernatural beings are thought to live in trees, and can be invoked for protection. One explanation states that the tradition derived from the Pagans who thought that trees were the homes of fairies, spirits, dryads and many other mystical creatures. Another explanation links the practice to wooden crucifixes. A more modern theory from folklore researcher Steve Roud suggests it derives from a form of tag called "Tiggy Touchwood" in which players are safe from being tagged if they are touching wood.