system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that washing bloodstained fabric with cold water helps to remove the blood, but washing with warm or hot water causes the stain to set?

It seems like most fabric stains come out more easily with hot water. Is blood just weird?
gpt
Cleaning protein stains can be thought of like cleaning a raw egg white off of a rag.

If you use hot water the raw egg white will cook enough to set into the cloth. You can't just rinse out the egg white if it's been turned into cooked egg white all imbedded around the cloth fibers. It's *much* harder to get out after the proteins are heated. Same with blood. 

That's also why you always want to check on a stain *after* it comes out of the wash. Because once you put it in the dryer it's probably never coming out. 

But for some things, like blood and grass stains, if you soak it with an enzymatic stain remover (like Shout™) for long enough the enzymes will eat the protein and it'll still wash out.

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*If you want to see what's happening on a more molecular level, take a handful of thin rubber bands. Gently mix them all together into a cohesive pile. Each rubber band represents a protein strand.

You could untangle this pile.

But not after you blast it with a hot hair dryer. That pile is not untangling ever again.