Why is hot chocolate so hard to clean out?

It never fails, every time I forget to rinse that mug out and I come back a couple days later and it's like you have to take a chisel to the mug to get the residual chocolate coco off. Even if you don't wait long, it's still a pain in the ass. I usually just fill the cup up with water and put it in the microwave and that softens it up enough and hopefully cleans any germs etc.

Why is hot chocolate so hard to clean out?
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