system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly are stains? Why do they stay on something seemingly forever? Are they remnants of the liquid that was spilled still on the clothing?

So I was at work today and noticed a coffee stain on the carpet. The boss said it was from 10 years ago. It had me thinking. Are there still particles and remnants of that coffee from 10 years ago? If so, why? What makes it stain?
gpt
All liquids (except pure things like distilled water) have chemicals dissolved in them.  If the liquid evaporates, the non-volatile substances in it will remain. Yes, the coffee stain still has particles from the coffee in it.  

Generally speaking, a stain is something that prefers to stick to the object (clothes, carpet, whatever) rather than be dissolved in water.  Coffee in particular has chemicals that are only barely soluble in hot water and almost insoluble in cold water, so spilling it on the carpet (which cooled it in the process) gave it a choice between sticking to the non-polar carpet fabrics or staying in the water as it was dabbed up.  If you brew some strong coffee and let it sit in a cup until it cools, you will see brown deposits on the side of the cup.  Same thing.

Some stains are from dyes in the product (i work with seeds that have a red dye in them, takes powerful cleaners to get it out of clothes or countertops), these are typically very hard to remove and take powerful cleaners which chemically alter the dye to a substance that dissolves easier, or a solvent that the dye prefers to be in more than whatever it’s stuck to.  This is the same as any other stain, it is just much more difficult to remove.