A knothole is a hole in a piece of wood where a tree branch once grew. When lumber is cut from a tree, the spots where branches attached often create these circular gaps or dark spots. If the wood dries out and the knot falls out or shrinks, it leaves behind a hole.

Knotholes matter in different ways depending on what you're doing. Carpenters sometimes avoid boards with large knotholes when building furniture because they weaken the wood. But in old wooden fences or barn walls, knotholes become something else entirely: natural peepholes. In classic stories and comics, curious kids press their eyes to knotholes to watch baseball games happening on the other side of a fence, or to peek into mysterious places.

A well-known knothole in American literature appears in To Kill a Mockingbird, where the children find small gifts hidden by their neighbor. That knothole becomes a secret mailbox, a connection between two worlds.

Knotholes show up in everyday language too. When something has a knothole, it suggests a small opening or gap where one shouldn't exist, like a weakness in an argument or a loophole in a rule.