Interesting that this was one of the most popular questions on Reddit's r/askscience subreddit two days ago; I wonder if PBS editors also trawl there for interesting topics.
https://old.reddit.com/r/askscience/comments/c6pjr3/why_do_p...
https://old.reddit.com/r/askscience/comments/c6pjr3/why_do_p...
A note of caution, the link contains British humour and liberal use of sweary words.
https://www.reddit.com/r/CasualUK/comments/8gtw49/spwm_final...