I was sitting in the right seat of a Cessna 172 recently, gazing into the distance while my student worked through his pre-taxi checklist, and I noticed that a beautiful white Cessna 210 on short-final had not extended its landing gear. We were on a ground control frequency, so I keyed the microphone and transmitted an urgent call.
"AIRCRAFT ON FINAL HAS ITS GEAR UP!" The aircraft descended below my line of sight behind a row of low T-hangers. I watched to see if the message made it from the ground controller to the lead controller. Moments later I saw the 210 climbing out and dropping his gear and heard, "Thank you," from the ground controller (I didn't get a 'thank you' from the 210 but he owes me.)
