Why does egg white turn white when cooked?

 A better question might be, "Why is it called egg white when it is clear?" (We didn't mean to imply, though, that your question wasn't brilliant; indeed, it is one of the best questions we received all day.)

The proteins in raw egg whites are folded into tiny little packets, repelling each other, and swimming in a sea of water. When they are heated, however, they bounce around out of control, unfold, begin to latch onto their neighbor proteins, and trap the sea of water between them in tiny nets. The molecules join together so tightly that they deflect the rays of light that formerly passed through the white, rendering it opaque (and white).

In answer to our own question, we speculate that the whites were named by the gentry, whose meals were cooked for them and never had occasion to see a raw, transparent egg white.

In an interesting aside, an in-law of ours (a male of a certain age) literally did not know that eggs come with one yolk and one white until he was newly married and his bride served him one fried egg for breakfast one morning. His mother had always served him two eggs at a time, and he suggested to his bride that economy was a fine thing, but they surely weren't so hard up that she had to serve him half an egg for breakfast.

True story, we swear.

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