system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does paper product (paper towels, napkins, etc.) become transparent when wet?
gpt
Imagine you are at a restaurant with a self-serve soda dispenser and a clear glass cup.   First, you fill the glass with that finely-crushed ice they have at soda dispensers.  Now look through the glass.  What color does it look?  White.  Opaque.  Like a white paper towel. You cannot see through the glass at all.  If there was an object (like a cell phone cover) behind the glass, the ice obscures it completely so you can't even tell what color the object is.

Now fill the glass with cold water (or Sprite™).  Most of the crushed ice is in there, but now the glass is translucent.  Like a wet paper towel.  Although you couldn't read a book through it, you could certainly see the color of an object behind the glass.

When ice and air are mixed in a glass, their very different indexes of refraction make light beams bounce around a *lot*, mixing everything up until you can't see through it.

When ice and water are mixed in a glass, their index of refraction is the same (in fact, if the ice itself didn't have air bubbles inside it, the glass would be almost completely clear and see-through).  So less bouncing of light beams.

The wood pulp and other material in paper has an index of refraction closer to water (and ice) than it does to air.  So the effect is the same.