A: Bob, can you tell me how many students passed the college entrance examination this year?
B: Off the top of my head, I'm afraid I don't know.
A: Never mind. Well, can you tell me where I can get the information?
B: Maybe from the Education Daily.
off the top of one’s head
exp. From memory or without much or careful consideration. （不假思索地，信口开河地）
e.g. She answered the teacher's question off the top of her head.
e.g. John can list the capital cities of all 50 states off the top of his head.
e.g. Mary: How much do you think this car would be worth on a trade?
Fred: Well, right off the top of my head, I'd say about a thousand.