It depends upon my mood and the situation. If I am out and about in the grocery store and I see strawberries with mold on them, I usually say something to the person who is about to grab them. If a stranger walks up to me and starts yammering away about their life (yes, for some odd reason, that has happened to me my entire life) I try to be kind while slowly backing away.
Understood on both counts. Mind you, I was a bit taken aback when a little German boy called me Opa, in a supermarket. His grandmother explained that he thought all old men were Opa.