I was stopped a couple months back and asked the same question. When asked the magic, “Do you know why I stopped you,” I answered I hadn’t a clue since I knew I was not speeding. Being courteous, respectful, and genuinely surprised earned me a warning for no tail lights or brake lights in a town that never gives warnings. Of course, mine was a legitimate stop for safety reasons. I’d have likely been arrested for asking why I needed to lower my window the rest of the way.
Do you know why I stopped you,
The best and most truthful answer to this question is ALWAYS, “No.”
LEO’s next angle: “Do you know how fast you were going?” Answer, “No.”
LEO’s next angle: “Do you know what the speed limit is?” Answer, “Yes.” Offer no further info.
In summary, use only the following responses in a traffic stop: Yes, NO and I don’t know. Offer no qualifying information.
If LEO want’s something else, ask, “Am I being detained or am I free to go?”
Trust me, if LEO wants to keep you, he WILL make up something (”I can smell alcohol.”).