Random Thoughts on the English Language #21: Guess... What?

Whenever I type "Guess what", I want to add a question mark at the end. And yet, it's really not a question. It's a command. Not sure if it's because it ends in "what", or what. Did some Googling and found that it's similarity to "Know what?", which could be expanded to "Do you know what?" might be part of what confuses me.