Short Answer: The letter "M"
Explanation: The letter "M" comes once in the word "minute," twice in the word "moment," but it never appears in the phrase "a thousand years." So, it’s a little mischievous letter that likes to play hide and seek! 🕒🔍 The sneaky "M" is always up to some linguistic tricks, but we’ve finally caught it in this riddle! 😄🎩