system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why is cereal (and dried milk) so hard to clean off a bowl if you let it dry?
gpt
It's mostly the dried milk to blame.  Milk is full of dissolved protein, the same sorts of molecule found in eggs and meat.  Dried protein makes a very good glue, and once it's dried it's very difficult to re\-wet and remove.  If you just ate the cereal with water \(an unpleasant thought\) and let it dry, then you'd just be dealing with some residual sugar from the cereal, which would dissolve in water no problem.