The raw egg is a combination of protein and water molecules. Protein has a very compact structure held together with weak bonding and that is why a raw egg is in a liquid state. When heated, these protein molecules react and make a complex structure. During this process, they hold water molecules and solidify. However, if heat is high enough and cooked for longer, some moisture evaporates and eggs start turning brown and sticking at the bottom of the pan. To avoid this, better use extra oil for greasing and avoid heat contact by stirring them at frequent intervals. Since sheet pan eggs are cooked on high heat for longer, it is sometimes unavoidable. Read more.