Protein Denaturation
Protein Denaturation is what happens when heat or acid makes proteins in milk unfold and clump together into solid chunks.
Think of it this way
A raw egg white looks clear and flows like water. When you cook it in a pan, the heat makes the proteins inside uncurl and tangle together. They form a solid white mass that cannot flow anymore. The heat changed their shape for good, just like heat changes milk proteins when you make cheese.
Explaining protein denaturation by grade level
When you add vinegar to warm milk, the milk turns lumpy. That happens because milk has tiny parts called proteins. Those proteins are curled up tight, like a ball of yarn. The heat and vinegar make them uncurl and stick to each other. The lumps you see are all those stuck-together proteins.