"You're not going to believe this..."

Hi Rafaela, If "Guess what?" means you have some exciting information to share, I'm not sure there is a *formal* way to say it, because it is kind of playful / joking, which isn't very formal. The other answers are mostly other informal ways to prompt someone to ask you about something. In terms of formal language, nothing really have the same playful spirit, but here are some phrases with similar purposes. If you want them to ask you what you know, maybe... "I have just heard/been told/learned/discovered something unbelievable." (If you don't explain, the listener will probably realize you want them to ask and ask you) If you want to know if they know...what you know, maybe... "Did you hear the news?" "Are you aware of the latest developments?" (I heard something, did you hear it? ~IMPLIED: exciting/interesting, or else you probably wouldn't talk about it) If you want to formally prepare someone for some exciting news, maybe... "I have a very exciting announcement to make..." "We have some wonderful news to share..." (this is the kind of thing that a company says when announcing a product to the public, or some new plan to its employees) Hope that helps!

Take a guess

You won’t believe… It’s unbelievable, right? Or What is your guess?

Have you heard the news?