Boiling an egg is a classic lesson in protein denaturation. When an egg is heated, its tightly folded proteins uncoil and link together, trapping water and turning the liquid into a solid. A common everyday 'mistake' is the green ring around the yolk of a hard-boiled egg. This is caused by overcooking. At high temperatures, the sulfur in the egg white reacts with the iron in the yolk to form ferrous sulfide. This chemical reaction results in a gray-green layer and a slightly 'eggy' smell. To prevent this, eggs should be cooled immediately in an ice bath after boiling, which stops the reaction. Additionally, older eggs are easier to peel than fresh ones because the pH level rises over time, causing the membrane to stick less tightly to the shell.