The answer to this riddle is "Envelope". An envelope is a flat paper container that is often used to hold letters or other documents. The word "Envelope" begins with the letter "E" and ends with the letter "E", but only contains one letter inside of it, which is usually a letter or some other document that is being sent through the mail.

The use of the word "letter" in the riddle is meant to be a play on words, as it can refer both to a single character in the alphabet and to a written message sent through the mail. By focusing on the latter meaning of the word, the riddle tricks the listener into thinking about a longer message that could be contained within an envelope, rather than the single letter that is the actual answer.
spam fails_task lang_mismatch pii not_appropriate hate_speech sexual_content quality toxicity humor helpfulness creativity violence