This riddle asks what can fill up a house while taking up almost no space. It sounds impossible if you picture furniture, boxes, or any other physical object.
The answer is light. Turn on a lamp or open the curtains, and light can spread throughout a room or even an entire house. It reaches walls, floors, ceilings, and corners without occupying space the way solid objects do.
That contrast is what makes the riddle work. “Fill up” usually makes us think of something that physically takes over an area, but light fills a space in a completely different sense. Once you interpret the phrase that way, the clue fits perfectly.
