This riddle asks: What can enter through a keyhole but fill the whole room?
The answer is: light.
A narrow beam of light can pass through a very small opening, including a keyhole. Once inside a dark room, that light can spread and illuminate a much larger space.
The clue sounds as though it is describing a physical object squeezing through a tiny opening, but light does not behave like a solid object.
That is what makes the riddle work. Something small enough to pass through a keyhole can still have an effect across an entire room.
