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

Disambiguated questions and answers:
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 do some explain the superstition saying knock on wood comes from involving protector beings trapped in trees and tapping wood for protection?
short_answers: folklore

Celtic or German folklore
wikipage: Knocking on wood

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 do some explain the superstition saying knock on wood comes from involving mystical creatures living in trees and touching wood to request good luck or distract the creatures?
short_answers: Pagans
wikipage: Knocking on wood

context: No context provided
question: What Christian religious pieces may explain the superstition saying knock on wood?
short_answers: wooden crucifixes

crucifixes

context: No context provided
question: Where do some explain the superstition saying knock on wood comes from involving tag?
short_answers: players are safe from being tagged if they are touching wood

"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
long_answer: The superstition that knocking on wood to avoid an unfavorable result after making a prediction has various possible origins. Some say it may come from 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. The custom might also be related to Christian religious pieces such as wooden crucifixes. It might also come from the game of tag, where some define a rule called "Tiggy Touchwood" so that players are safe from being tagged if they are touching wood.