“Guess who I saw today!”

Today when you were at the grocery store you ran into someone who you didn't expect to see - your friend from college who you haven't seen in several years. You were excited to see her. When you get home, you want to tell your husband about it. You say:

Guess who I saw today!

Guess (something)!

People use this phrase when:

• They're excited about something.
• They want to tell someone about it.
• They want to build the suspense before they talk about it. (Meaning that they want the listener's excitement level to rise.)

When you say "Guess who I saw", it lets the listener know that you saw someone unexpected and exciting. It's more interesting than just directly telling them who you saw. Here are some other examples of phrases like this:

Guess what I heard!

Guess where I'm going!