I was just now humming to myself on the streetcar at the very back (we have those super elongated modern three-car streetcars) and some kid got on with a form-fitting pink mask and sat near me. I know he was observing the old person, maskless, humming, so I didn't stop. Maybe worried about sound waves hitting him like a covid-arrow, I don't know. Or marvelling at my naïveté. :-)