S is for Squawk
A squawk is a signal sent out by a Transponder that lets an Air Traffic Control Center know where you are on radar. You would be seen on radar anyway, but the Transponder actually identifies you by a code attached to the little airplane blip on the radar screen.
You've probably heard a lot lately about how the Transponder had been turned off on Malaysia Airliner Flight 370. When the pilot (or whomever) turned off the Transponder, air traffic controllers lost all contact with them. Flight 370 became one of hundreds of unidentified blips on a radar screen.
When flying I.F.R., as airliners do, that blip allows air traffic controllers to know the altitude at which a plane is flying in addition to its location, which prevents mid-air collisions.
When Mike and I were making an International Flight into Saskatchewan in our RV7, we were required to use our Transponder. We were given a 4-number code that was picked up first by Seattle Air Traffic Control Center, and then Salt Lake Aircraft Control Center, and then Manitoba Air Traffic Control Center, as we successively passed out of each center's jurisdiction.
In the image below of the instrument panel in our RV7, the Transponder is the instrument right above the radio. The radio is the instrument with the green display that reads 1200.
What do you think about the mystery of Flight 370? What do you think happened? It's certainly the oddest airline disaster story I've ever heard.