system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does a brown paper become semi-transparent when saturated with oil but not when saturated with water?

I have some assumptions (like maybe oil fills in gaps between fiber where as water makes the fibers swell instead?) but I’d like to know the answer and the details of why. Thank you!
gpt
When liquids saturate paper, they fill tiny pockets of air inside of it. Those air pockets "exhaust" light quite efficiently (or more accurately, scattering occurs at the paper-air phase boundary, but that's above ELI5). Imagine cycling for 10 feet, then dismounting, walking 10 feet, mounting, cycling 10 feet, dismounting etc etc until you've travelled 100 feet. It will be much easier if you simply cycle 50 feet then walk 50 feet. The photon loses much less energy if it doesn't have to "mount" and "dismount" all the time, which is what the liquids in the paper allow by removing all those air pockets, leading to translucency when the photon makes it all the way through the paper. 

As for water vs. Oil, they will both wet the paper, but only oil makes it go translucent, where water will just darken the patch. This is because oil has a similar refractive index to the paper, which means a smooth ride for the photons. Water has a refractive index quite different to paper, meaning you essentially have the same problem as the air pockets. 

Furthermore, some brown papers have a waxy coating which would stop water wetting the paper, but would not stop oil from wetting it.