Why do eggs change when cooked?

When an egg cooks, heat makes its proteins unfold, then fasten to one another. That new web catches water and turns the slippery egg into something firm and cloudy. Nothing new is added; the same egg has been rearranged by heat.

The fun detail is that cooked egg white is really a tiny scaffold you can eat. The proteins were once folded so neatly that the white looked clear. After heating, they spread out, link up, and scatter light, so your breakfast suddenly looks white.