system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes some clothing materials "warm when wet" (like wool) but others not (cotton)?
gpt
In the case of wool and cotton it is the individual strands that make the difference.

 Wool is rather none porous (it's just hair) and ends up coating itself with natural oils (from your skin, or if raw, the sheep's) that help repeal water. It is a very curly/crinkly material and this creates many pockets of air that warm up and act as insulation.

Cotton is a porous plant fiber (absorbs oils and water into it internal structure) that is significantly straighter. The straightness of the fiber helps create a smoother fabric that lays against the skin rather than poking into it (not itchy, yea!) but is less effective at creating air pockets for insulation.