When I'm alone most of the time I just respond with something like a joke and laugh it off as it doesn't bother me most of the time.

On the other hand I got to use this when my son was with me: "Nathaniel, this is an example of your typical ignorant, ill-mannered, human. Learn to spot them and avoid them as much as possible".

I have only had to use it once and the effect was exactly as you could imagine. Stunned silence combined with utter embarrassment and a deer-in-the-headlights look that was epic.