What can fill a room but takes up no space?

How to think about it

1. Anything with volume, like water or boxes, would take up space, so the answer has no volume.
2. Think of things that spread through a room instantly: sound, smell, light.
3. The hint says to flip a switch. Which of those appears when you do?
4. Light fills every corner of a room and leaves the room exactly as empty as before.

Why “Light” is the answer

When you turn on a lamp, light reaches every part of the room at once, yet the room has no less space for people and things. Light has no volume in the everyday sense, which is what the riddle means by taking up no space. Sound and smell also fit the words, but the light-switch hint settles it.

Common wrong answers

• Air. Air fills a room but does take up space; that is why you can compress it into a tank.
• Sound. Sound also fills a room without using space, so it is a fair alternative; the hint points to light.