A knot? Tell me about it.
When a tree grows it will have branches on it. The wood in the branch is at right angles to the tree.
When you saw the tree for boards, the branch shows up as a round area at a right angle to the board. That is a knot.
I saw a piece of coal that had that in it, so that shows that the coal came from a tree.
You may have thought I was talking about a knot like a knot in a rope. I wasn't. It was a knot like a knot in a board.