我们都知道打招呼用
- How are you? 
- Fine, and you?
或者
- How's it going? 
- OK. How's it going with you?"
除此之外
我们在美剧中也会经常看到
朋友见面用 6park.com
What's up?
来打招呼的 6park.com

遇到这句话
又该怎么回应呢？
我们先来一起认识一下
“What's up?”的意思是什么

(something) is up means something is happening. "What's up?" is just another way of saying "What's happening?" / "What's going on (that might interest me the speaker)?"
Sth is up意思是sth is happening（某事）发生。“What's up?”其实是“What's happening?（发生了什么）”，“What's going on(能勾起发问者兴趣的事情)？”的另一种形式。


That is an expression that has about as much meaningfulness as "How are you?" or "How's it going?"  6park.com
这是一个和"How are you?"、"How's it going?" 基本上有相同意义的表达。
All, including "What's up?" are used as greetings. 
它们所有的，包括"What's up?"都可以用作打招呼。
It has however fallen into common usage both in the US and UK.
它在美国和英国都已成为常用用法。
如何回应“What's up?”
就问候这一层面来讲，你的回复只是需要简单的回应它的打招呼，或者是说明情况一切正常，没有什么特别的事情发生即可。

-What's up? 
忙什么呢？
-Not much / nothing, how about with you?
没啥，你呢？
还有一些情况就是你身边确实发生了些什么，或者你手头正在做什么，你就可以直接交代出来，

比方说你正在洗车，然后有人见到你问候你 6park.com
-What’s up?
忙什么呢？
你就可以说
- Not too much, just trying to get my car clean.
没什么，就是清理清理我的车。
又或者你正在街边散步
有人过来跟你打招呼
-What’s up?
忙什么呢？
你就可以说
- Just trying to get a little exercise in. What’s up with you?
就锻炼一下，你呢？
好啦，今天的小知识点，你学会了嘛？ 6park.com