What comes once in a minute, twice in a moment, but never in a thousand years?

How to think about it

1. Nothing physical happens once a minute, twice a moment and never in a thousand years, so the counts must be about the words.
2. Look at the spelling of “minute”: one M.
3. Look at “moment”: two Ms. Look at “a thousand years”: no M at all.
4. The letter M fits all three counts exactly.

Why “the letter M” is the answer

The letter M appears once in the word “minute”, twice in “moment” and not at all in “a thousand years”, which matches the riddle’s counts perfectly. The question dresses a spelling observation in the language of time so you look at clocks instead of letters. Versions of it have been passed around for a very long time, in print and by word of mouth.

Common wrong answers

• A heartbeat. Hearts beat far more than once a minute, and they certainly beat during a thousand years of history.
• The letter E. E appears once in “minute”, twice in “moment”, but also in “thousand years”, so the last clue fails.