Q: What is a thermocouple?
A: A thermocouple is a highly advanced, extremely complicated, and utterly fascinating device used to measure temperature. No, really, it's just a fancy name for a temperature sensor.
A: 3. Yes, 3! There's the B-type, the E-type, and the... actually, just kidding, there's really only 2. And also, who cares?
A: A thermometer is like that old, boring cousin of thermocouples. It just shows you the temperature, whereas a thermocouple has feelings, it has personality, it has... okay fine, they're basically the same thing, but the thermocouple is more exciting.