It’s not just the title of a 1980s Brat Pack movie: St. Elmo’s fire is the name given to bright, sudden flashes of apparent lightning that can dance across a cloudy sky when thunderstorms are nearby.

When Elmo decided to check the emotional pulse of the internet on Monday and ask, “How is everybody doing?” the fiery-colored Muppet couldn’t have anticipated the seismic response he’d receive. Elmo’s ...