Sharing the puzzle on Twitter, the teacher explained: "The first guess from one of my 1st graders was 'death' and such an awed, somber, reflective hush fell over the class that I didn't want to tell them that actually the answer is the letter e, which just seemed so banal in the moment."
He added: "Before I finally revealed the 'correct' answer to the riddle, to a largely unimpressed audience, I fielded other guesses that continued along a similarly existential vein.
"There was 'NOT everything', 'all stuff', 'the end', and maybe my favourite, 'nothingthing'."
The teacher's story has since gone viral on Twitter, receiving more than 73,000 retweets.
Social media users were impressed by the answer "death", saying it was a "better answer" than the letter e.
One person commented: "See, it's things like this (as well as my son's insights) that give me a lot of hope for the future".
Another added: "Gen Z are all existential nihilists aren't they".