Why is this the answer?

The answer is keyhole because this clever riddle uses a fun word trick to make you think of something that moves, when the answer is actually an empty space. A keyhole is a very important little part of a door, but it is not a thing that can walk or fly. Let us look closely at the first part: "What goes through a door". When you think about a keyhole, it is the small, perfectly shaped slot cut right into the middle of the door. This little hole is a passageway that connects one side of the door to the other, like a tiny tunnel. So, in a way, the keyhole "goes through" the door, allowing you to peek through or pass an object, like a key, from one side to the other. It is a fixed path that leads from one room to another, or from the inside to the outside. Now for the tricky part: "but never goes in. And never comes out". This is where the riddle is so smart. Things that go in usually come out, like you going in and out of your house. But the keyhole is not an object that can move. It is simply a hole, an unchanging feature of the door itself. A hole cannot pick itself up and walk away. The hole is a permanent part of the door's structure, which means it never moves or changes its position. It does not "go in" because it is already a part of the door, and it does not "come out" because it is a fixed opening. It is always in the exact same spot on the door. The keyhole is a patient helper whose real job is to guide the key so you can safely lock or unlock the door. It is always there, waiting in the same spot, ready to let the key do its work. It is a small but mighty hole that is always passing through the door without ever having to move a single inch of its own. This is why the keyhole is the perfect answer to this funny and imaginative riddle.