system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does wool keep you warm when it’s wet but cotton doesn’t?
gpt
The shape of wool fibrils creates air pockets in the large- scale structure  of the fabric even when the fibers are wet.

And the structure of the fibers allows for then to get wet,  but the moisture is wicked to the center of each fiber,  so the surface stays drier than the equivalent area of a cotton fiber. 

These pockets provide insulation and are absent in wet cotton.