Why are things transparent?

Answer 1

Because the light waves travel through its molecules.

Alternative answer:

Light waves have a dual nature; one electromagnetic and another a photonic wave. These waves travel through air, hitting all kinds of objects. Some of these objects are transparent (glass), which allow the light waves to pass only in the form of photonic wave. After the light wave pass through the object (glass), it obtains back its dual nature.

Answer 2

Transparency occurs when materials do not absorb light at certain wavelengths and instead allow it to pass through without being scattered. This happens when the atomic or molecular structure of a material does not cause light to be absorbed and reflects it in a way that is not visible to the human eye.