Simple: oats are up to 60 per cent starch, which is a thickening agent. Starch is a carbohydrate that forms granules made from polymers called amylose and amylopectin. When you cook oats in water or milk, the starch granules swell to absorb liquid and the porridge starts to thicken.
When porridge cools, the amylose and amylopectin polymers become less energetic. This means they interact with each other to expel water and form a stronger scaffold. And, as the freed water evaporates, the porridge hardens.
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