I've seen many answers in various threads, but not one dedicated to quite possibly the most popular question out there.

here is my best answer:

"My wife's lipstick"

and yes, there is a story behind it.

I was attending a homecoming game at my alma mater, and visited my old fraternity house. Of course I was wearing my kilt, which prompted many questions from the current members. While there, I bumped into a fellow alumni who struck up a conversation about my kilt. Apparently he also wears kilts regularly, and his wife was his most active supporter of them. We soon began swapping answers to "the question". He and I were going back and forth with the usual answers, and his wife interjects. She tells a story of how they were out at a bar one night, he in his kilt, and a shapely woman approached them. She asked the question with "an inquisitive sparkle in her eye"( quote from the wife) and reach for his hem. His wife pushed her hand away and told her "His wife's lipstick". The questioning woman quickly turned away, embarassed.

Suffice it to say, during her story he stood tall, proud of his wife's answer.

I would too. Best answer I've ever heard. Especially from a wife.