system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do many things appear darker when wet?
gpt
Because when something is wet, light bounces around inside it more (as opposed to merely bouncing off the surface) before being reflected back to the eye. The more the light bounces, the more of it gets absorbed, the less reaches the eye, and the darker the object appears.  
  
The reason the light bounces more is that the moisture increases the average scattering angle of the light particles. When the photons strike the surface of the wet material most of them bounce forward and hence deeper into the stuff rather than backward toward the eye.