Many years ago I was working at a summer camp. I was talking to a female counselor about something so boring that I have forgotten what it was. What I do remember, though, was that at some point in our conversation I made her laugh.
Immediately after we finished talking and parted ways her campers ran up to me and said that they knew we were dating. I told them that wasn't true because it wasn't but they wouldn't hear it. They had proof, they said. So I asked them what made them so sure and they said they saw her laughing and that could only mean one thing.
You can't really argue with that and rumors have been started for a lot less.