Coagulation
Coagulation is when heat or acid makes a liquid protein turn firm and solid, like an egg cooking in a pan.
Think of it this way
When you crack an egg into a hot pan, the clear liquid white turns solid and white as it heats up. The loose proteins in the liquid link together and form a firm, set mass. Acid does the same thing — lemon juice in egg whites makes them firm up too. Heat or acid locks the proteins in place, changing the egg from a liquid to a solid.
Explaining coagulation by grade level
Think about cracking a raw egg. The white part is clear and runny. When you heat it in a pan, it turns white and firm. The heat changes the egg from a liquid to a solid. You can not undo it once it cooks.