✔ 最佳答案
What's up? = 有什麼事?
通常人哋會係對話時, 一開始就說出來.
Case I:
A: Hi, John.
B: What's up?
A: Nothing, how are you?
B: Fine, how about you?
...
Case II:
A: Hi, John.
B: What's up?
A: Can I borrow you accounting reference book? I need it for my homework.
B: Sure, I will bring for you tomorrow.