"Oh, I saw you and your husband on your tandem last night."
"Oh yes?" (in a vague non-committal manner. People seem to like to tell me when they've seen me on the tandem. I'm not sure why, or what response is expected, but it seems to make them happy.)
"Yes, I was just behind you at the lights. Just one thing," (mock scolding) "Where was your helmet?!"
"With my mountain bike, for the next time I need it."
"Well, it meant I took extra care not to knock you off as I went past because of it!"
And that's why I don't wear a helmet on the road, folks!