Our idiom today is you've got me.
This means you don't know the answer to the question.
A. What is my sister's name?
B. Hmm, you've got me. (Can also be said as "you got me")
A. Do you know who won the Gold metal in the 100 meters, at the 1936 Berlin Olympics?
B. That's a good question. I don't know. You've got me.