A: I was wondering if you could give me a day off.
B: What for?
A: I'll have to rewrite my thesis. That will take much time.
B: Stop trying to fool me. I have your number.
have one’s number
exp。 To understand or know someone or something well enough to anticipate what they will do。 （心中有数,知道某人的底细）
e.g. Oh, don't worry — I won't let him cheat me. I've had his number.
e.g. Oh, don't worry, I've had his number—he's a typical brat.
e.g. I have his number. He enjoys less popularity than most other professors.
take much time：花费很多时间
What for? 为何？为什么？
give someone one day off：给某人一天的请假