Why Do We Knock On Wood?

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Why Do We Knock On Wood?
Knocking on wood is said to bring good luck.

There are a lot of little things people do to bring some good luck. 

Some people think carrying a rabbit’s foot or finding a four-leaf clover can help them get some good luck.  

Michael Jordan wore his college basketball shorts under his uniform during his career because he thought they were lucky. 

And some people knock on wood for good luck. 

But where did the habit come from?