What can pass through glass without breaking it?

How to think about it

1. Solid objects would shatter the glass, so the answer is not an object.
2. The hint says to look at a window. What comes in through a window all day without opening it?
3. Sunlight passes straight through clear glass; that is why windows are made of it.
4. Light goes through glass and leaves it unbroken.

Why “Light” is the answer

Clear glass is transparent, which means light passes through it almost completely while the glass stays intact. That is the whole purpose of a window, and the riddle just asks you to name the thing a window lets in. Sound and heat also travel through glass to some degree, but light is the answer the hint points to.

Common wrong answers

• Sound. Some sound does travel through glass, so it is a partial fit; the window hint points to light.
• A bullet. A bullet passes through glass by breaking it, which is exactly what the riddle forbids.