This wordplay riddle asks: what has a dozen letters but only one word?
A common answer is a mailbox.
A mailbox can contain many letters at the same time, even a dozen or more. The trick is that “letters” is being used to mean pieces of mail rather than characters of the alphabet.
The “one word” part points to the word “mailbox” itself: it is a single word describing something that can hold many letters.
The riddle works because it switches between two meanings of the word “letters.” Once you stop thinking only about alphabet characters and think about posted letters instead, the answer becomes much easier to spot.
