Why does an egg turn solid when you cook it, and can you un-cook it?

An egg turns solid because heat unfolds its proteins and they clump together. Cooking an egg makes the proteins in the yolk and the white denature and solidify. In the raw egg each protein is folded into a compact shape held by weak bonds, which heat, acidity and other stresses can break. Once unfolded, the hydrophobic parts of the proteins stick together and form a network, a gel. That is called coagulation.

Heat is not the only way. Pouring egg white into acetone also turns it translucent and solid.

Different proteins unfold at different temperatures, which is why an egg can be cooked to different textures. Egg white proteins begin to coagulate at 55-60 °C, while yolks thicken at 65 °C and solidify at 70 °C. The yolk never solidifies before the white, and in a boiled egg heat takes longer to reach the yolk through the white.

Can you un-cook it? In principle: the process may be reversed by breaking the connections between the proteins, which has been demonstrated with sodium borohydride or vitamin C. For a cook, though, the practical point is timing: Heston Blumenthal published a recipe for the perfect boiled egg after relentless trials, starting the egg in cold water barely covering it, taking the pan off the heat when the water bubbles, and waiting six minutes.