I've had to explain to several people about the difference between the "white smoke" and the "black smoke".
The best of the cuff comeback I've heard was from the late George Hart. Smoke was rolling out of his #972 as she was being fired up one day, and a woman came up and was reading George the riot act about the pollution.
George's reply? "Well, I'll park my steam engine in the engine house with the fire banked and I'll sleep in there with it tonight. You go home and park your car in the garage with the engine idling and you sleep in there with it tonight, then we will get together tomorrow and talk about pollution."