The complicated stuff:
Electricity is comprised of a current which is pushed through a conductor by a voltage in it's simplest terms. All conductors have a quality we call resistance which resists the flow of electrical current through it. When we push too much current through a resistance, the result is Heat.
Now go a make a piece of toast to enjoy, and while the bread is toasting, take a peek down inside the toaster slots. Those little rows of fine wire are working their magic and toasting your bread an even golden brown for you to enjoy. Too much current is flowing through the fine resistance wire the coils are made from, and they are glowing red hot because of it.
Smart switches:
But how did the current know when to flow? How did the toaster know when you put the bread in it and when to pop?
Long before there were smart bombs, there were smart switches for our toasters. And these little marvels truly ARE "smart" switches, not just primitive timers that expire at a given time, they are far more educated than that.
Most of them rely on an ingenious little piece of metal called a bi-metal. As the name implies, it is two pieces of different metal fused together, creating a piece of metal that when heated expands at one rate on one side, and another rate on the opposite side, forcing it to bend in one direction.
The Miracle:
So here's your toaster in action. You put the bread in, and push down on the handle to engage the toaster. This closes the switch that allows current to flow through the heating coils that toast your bread. This also allows current to flow through another tiny heating coil that you don't see, and which is wrapped around that bi-metal we just talked about.
As your bread begins to toast, the little bi-metal begins to bend in one direction. This is the "heat up" stage of the toasters cycle, and when it is completed by the bi-metal bending far enough in that direction, a click will take place, stopping the current flow to the bi-metal heater.
The bi-metal will then begin cooling, and therefore bend back to its' original position, at which time it will "trip" the mechanism that holds the main carriage down for the toaster, and your golden brown toast will be ready for your enjoyment.
There are other designs out there, some of them quite sophisticated, but for decades the one described has made up the backbone of the electric toaster industry for most major manufacturers.
Some trip with a mechanical arm, some trip with a solenoid, which is a coil of wire with a metal plunger in the middle of it, some have another plunger that serves to slow down the popping so the bread doesn't fly into the sink when it pops.
How we take the little things we all rely on for granted, yet they truly are little marvels in their own right.
