A few years back I was in a ground school being taught by a freshly minted instructor. He was explaining that on downwind at an airspeed of 100kts if there was a tailwind of 20 kts the aircraft would have an airspeed of 120kts. During the break he joined me outside for a smoke, I took a cigarette paper and threw it into the wind and asked him if that wind was 10kts what is the papers airspeed? He said 10kts, I gave him a raised eyebrow and went back inside. After the lunch break (where he did not join myself and the other students) he came back into class looked over and just nodded with a sheepish look on his face. He proceeded to review the mornings session and correct any misapprehension any of us may have had.