Why do eggs become solid when you cook them?

We were cooking scrambled eggs (Liem's favourite breakfast) and Liem asked "Why do eggs become solid when you heat them. Things like ice or butter melt when you heat them." Good question! I thought of "entangled protein chains" straight away but then remembered a talk given by Herve This (French molecular gastronomist) who said that our "feory is wong!" I looked it up and it's because when you heat eggs, you made sulfide-sulfide bonds between the protein chains. Sulfide-sulfide bonds are the same bonds that make hair curly. So, if you can break the sulfide-sulfide bonds (eg with sodium borohydride, don't try this at home) you can "uncook" an egg and make it liquid again which Herve did in the lecture that I attended. I looked this up the next day after Liem asked the question and tried to explain it to Liem, but he'd lost interest by then, "I don't care about eggs anymore"...