what = the thing/things that,因此what可以用于定语从句,但不需要先行词,换句话说,what其实引导的是名词从句.比如：
This is what I want.= This is the thing that I want.
这就是我想要的.
Do you know what they're doing?= Do you know the thing they're doing?
你知道他们在做什么吗?
What caused the accident were stones.= The things that caused the accident were stones.
一些石头导致了这起事故.