Thermocouples are cool little items, conductors which generate a voltage when subjected to a thermal gradient (Wiki). That's the old thermoelectric effect that you remember from high school physics.
The effect was accidentally discovered by the Estonian Thomas Seebek in 1822. Most of our general-use thermocouples are the inexpensive K-type, Nickel-Chromium and Nickel-Aluminium.
I had the unfortunate opportunity to learn more about our dependence on these mechanisms over the weekend. Among hundreds of other things they are used for, I learned that they control the pilot light on gas water heaters. If the pilot light goes out and your thermocouple sensor is on the blink - no hot water.