Keep somebody guessing
Definition 1 of 1
Pro Tip 1/3Uncertainty
The phrase implies that the outcome or future actions are unpredictable and not obvious. 
The magician's act was so intriguing; he kept us guessing how he did everything.
Pro Tip 2/3Suspenseful Tone
Used to describe situations where suspense is created intentionally, often in storytelling. 
The cliffhanger in the TV show's season finale really kept everybody guessing.
Pro Tip 3/3Avoid Using Literally
The phrase is not used to mean that someone is literally thinking of guesses, but rather they are uncertain. 
She said she had a surprise for me and kept me guessing all day.