A: I heard that Rose would be your headmaster.
B: Who told you that?
A: It's bob.
B: Bob? he always talks off the top of his head.
off the top of one’s head：不假思索地隨口而說
e.g. I can't tell you the answer off the top of my head.
e.g. She answered the teacher's question off the top of her head.
e.g. When Lorraine was asked to recite, she talked off the top of her head.
e.g. You know, I didn't study for this English quiz. So when the teacher called my name, I was frozen. Then standing up to recite the poem, I found myself doing it off the top of my head! Can you believe that?
e.g. Sorry, I can't give you the exact figure off the top of my head. But I guess the percentage of working women in the American labor force right now is about 47%.