To me, sentences that start with "Guess" are in the imperative mood, thus, should end with a period:
- Guess who's coming to town.
- Guess what we had for dinner last night.
Why do a lot of publications put a question mark at the end of these sentences? I've been taught that you put a question mark only if the sentence is in the interrogative "mood". In these cases, we're not asking who's coming to town or what we had for dinner last night – we know who's coming and what we ate. We're telling that second person to guess.
If "Guess _ ?" is correct because it expects an answer just like a regular question, is it acceptable, then, to write:
- Tell me who's coming to town?
- Tell me what I had for dinner last night because I forgot?
"Tell __ ?" looks just as unsettling to me as "Guess _?".
Best Answer
Because you have been taught an oversimplification.
Most English speakers would have no idea what you were talking about if you mentioned the "interrogative mood". People put a question mark on the end if it feels like a question.
Conversely, a polite order like
or a less polite one like
are often written without question marks, because although syntactically they have the form of questions, they are not in fact questions at all.