Knowing which mask is best to prevent person-to-person transmission of the COVID-19 coronavirus is still an open question for most, but slow-motion video shared on YouTube early in July provides a visual basis to judge how best to keep safe amid the global pandemic. Professor Joe Hanson of the YouTube channel "It's Okay To Be Smart" released a video earlier in July, reviewing the basics of why people should wear masks — in a visual review and primer and recent studies on the effectiveness of masks to prevent the spread of the COVID-19 coronavirus. It's rare for sneezes and even coughs to be visible to the naked eye — which happens when oral expulsions move through a ray of light. Hanson and his team decided to demonstrate how breath leaves the body using the Schlieren imaging technique combined with slow-motion cameras set to roughly 250 frames per second.