Evidence that birds sleep in mid-flight
I'm reminded that we still don't know conclusively what purpose sleep serves[1]. We know about things unimaginably distant in scale or time -- like quarks and the Big Bang -- but we don't know the answer to something seemingly pedestrian.<p>[1] <a href="https://en.wikipedia.org/wiki/Sleep#Functions" rel="nofollow">https://en.wikipedia.org/wiki/Sleep#Functions</a>