The answer is the letter "m".

• It appears once in "second" (no, wait—actually, "second" doesn’t contain an "m").
• It appears twice in "moment" (moment).
• It does not appear at all in "a thousand years."

Wait—let me correct that: The classic riddle is usually phrased as: "What appears once in a minute, twice in a moment, and never in a thousand years?" The answer is the letter "m".

But in your version, "second" doesn’t contain an "m." If we stick strictly to your wording, there is no letter that fits perfectly. However, if we assume a typo and you meant "minute" instead of "second", then the answer is "m".

If the riddle is exactly as you wrote it: There is no letter that appears once in "second," twice in "moment," and not at all in "a thousand years."

But if it’s a variation of the classic riddle, the answer is m. Did you mean to say "minute"?