Hinge

A typical door has three parts: the pin, the knuckles, and the leaves. The two leaves touch the surface: one for the door, one for the frame. The knuckle's where the leaves meet in the middle. It lets the door move once the pin's slid in, to hold the thing in place. The leaves are also known as wings. When a door is closed, the wings touch, pressed perfectly together: door to hinge to frame. The knuckles are hidden on the inside, then. From the outside, you might think the door is floating. To open the door, you first need a body. The identical wings will part. The door will retreat to clear a space. Then, through the frame, our bodies might move.