Is It A Print Or Is It The Original?

twice this month I have been asked this question while selling my photographs. I point out these are photographs (in case they think they are paintings) but that doesn't satisfy them. I don't know how to reply. I guess it probably doesn't matter as the question means they almost certainly aren't going to buy - but my explanations seem to leave them thinking I am lying, or cheating or defrauding them - anyway they go away unhappy - I was polite, I explained the nature of photographs and original negatives, slides or digital files - but that doesn't cut it - oh well.