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Okay, at approx 310 degrees Farenheit, the Maillard Reaction starts (which means, basically, that starches and sugars start to caramelize {turn brown}). This gives toast its unique flavor.

If you heat the toast more, the grain of the toast starts to turn to carbon. You've burnt your toast.

Doh!

They actually used to use fire to toast bread, right? Wow. But such things were unnecessary in the electric age. The amount of heat a metal gives off when electricity is applied to it is related to its resistance. This resistance is what actually causes heat. Usually an alloy does best, like nickel-chromium, AKA Nichrome. Sometimes a coil of Nichrome is used, other times it's wound on mica strips.

There are three types of toasters, Automatic Toasters, Manual Toasters, and Semi-Automatic Toasters, capable of shooting toast at a high rate of speed.

The mechanics of these are pretty forward, a manual required user intervention to save the toast from turning to carbon. A semi-automatic had a bell, but did not spring up the toast. A fully automatic toaster uses a thermostatic or timing switch to eject the bread.