system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Although water is clear, why does it make certain stuff appear darker when wet (i.e clothes, towels, paper, etc)?
gpt
The reason is not that water is not transparent enough, as another user suggested. But on the contrary, it is that the materials are slightly transparent. Let me explain. If you take, say, a sheet of paper, it seems to be quite opaque, right? However if you put this same sheet of paper under the microscope, you will see individual fibers, those little specks of cellulose, and if your magnification is high enough, you will notice that they are somewhat transparent. Not quite clear, quite cloudy actually, but still semi-transparent.

Now, you don't notice it usually when you look at the sheet of paper. But the light that falls on it gets refracted and reflected from these cellulose fibers, and gets trapped in the air cavities between them, and gets scattered, and finally reflected to your eye. So the sheet looks rather light. Same with stone (which consists of semi-transparent crystals, or sand grains), or fabric (with individual micro-fibers), or wood.

However if you make the surface wet, water will fill these little air pockets between the fibers, grains, crystals, of whatever your material is composed of. Where it was air, you'll now have water. It will change the optic properties of these surfaces: you will have less refraction, and less reflection. If you ever put a glass cup underwater you might have noticed that things made of glass are less noticeable, less visible underwater than they are in the air. Same happens here with your surface. The micro-pockets of air that used to scatter the light, making the surface lighter, don't scatter it as well anymore. The light is not sent back into you eye, but goes deeper into the material instead, and gets absorbed there. And it looks darker.

When there are no tiny pockets of air (a mirror, or a polished table) it does not get darker when wet. When the material is not semi-transparent even at a microscopic level (say, a brass knob, or a steel knife), it also doesn't get darker. And now you know why =)