What causes coffee to stain?

Answer 1

I can't say exactly, but things stain because the molecules in the stain are more strongly attracted to the molecules of the material than the molecules of non-staining things.

Coffee contains volatile oils that can remain in fabric and most other surfaces. Any porous surface will trap those oils and hold the dark color.

this is wrong

Answer 2

It turns out that liquids, such as coffee, that are made up of spherical particles dry differently than liquids with more oblong particles, scientists have found. In the case of coffee, each particle, or clump of molecules, is roughly a micrometer (one millionth of a meter) wide, which is about 10,000 times larger than individual coffee molecules, researchers said.

Answer 3

To remove a coffee stain, use a commercial stain remover according to the directions on the bottle. For a greener and less expensive method, try using baking soda and cold water on the stain.