system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes some objects clear (glass, water, etc.) and others opaque? What goes on at the atomic level?
gpt
Atoms are 99,99...9% nothing inside. So let’s assume that all stuff is transparent. Then you should ask yourself the question why certain things are opaque.


Well, it has to do with the reflection and absorption of light. Light travels in ‘packets’ of certain size depending in its color. (Technically the color is a result of its size, btw). 

Now let’s assume that the atom is a sieve on the inside. If the holes of the sieve is smaller than the packets, they won’t go through. But if the holes are bigger, the light packets will go though. 

Different materials have different hole sizes of their sieve. Opaque material has very small holes, preventing any lightpackets from going through.