You can be out, just mindlessly strolling down the sidewalk, perhaps even with the slight tracings of a smile on your face, when suddenly you overhear someone spew something so profoundly clueless that you're stopped in your tracks. You might stand there, paralyzed in troubled wonder as you try to figure out how someone could ever end up so lost. But then, you either let it go (recognizing that everyone says something dumb at one point or another), or actually try to let them know what's up. Of course, if you choose the latter approach, just be ready for defensiveness. Such a response tends to accompany blind ignorance.