The phrase “guess what” is often spoken with a raised tone (or pitch) at the end of the phrase much like a question. For this reason, people sometimes punctuate it with a question mark.
- i.e. Guess what?
However, in the strictest sense and following the rules of English grammar, the phrase is a command. We are beginning the phrase with an unspoken, but understood “you” followed by a verb in the command form (guess) with a direct object pronoun (what).
This alone makes it a command and as such it should be punctuated with a period or an exclamation point.
- i.e. Guess what.
- i.e. Guess what!
The reason that we can and do say this phrase with a raised tone is because we are often excited when we say it. One expresses excitement in English by raising the tone of one’s voice. This change in tone helps the listener to pick up on your excitement.
When we command someone to guess something, it is often done with excitement because something unexpected has happened. Thus, we raise our voices to express that excitement and that makes the command sound like a question.
However, the phrase “Guess what” is not always said with excitement. All one has to do is to think of the classic Winnie the Pooh character, Eeyore, to understand this. In his gloomy voice, Eeyore often says “Guess what” without a raised tone and everyone knows that he is never excited.
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