Using 何を with intransitive verbs (eg 何をにやついているの?) > … > What are you grinning at, you bastard? This sentence (asked in this question) seemed fairly natural and simple to me...until I realized ** is an intransitive verb**. [×] is ungrammatical. Instead, you have to say something like . Then why is the first sentence ever possible? Well, after some pondering I came up with a similar expression: > So apparently these with intransitive verbs mean _why_ rather than _what_. But is that really true? I found a page where one person says can mark a reason/trigger. But I'm not totally convinced, since all I could find was this short comment. I'd appreciate if someone explain this or point me to a longer article/reference. Should this be considered as a fixed expression like English "how come"? (Just to be sure, as a native speaker of Japanese, I know these examples using are natural, but I'm just having trouble getting a nice grammatical explanation.)

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