9.64 ellipsis in questions

You can often use ellipsis in questions when the context makes it clear what is meant. The question can consist just of a 'wh'-word.

Note that you can also use 'why not'.

Note also that you can use a 'wh'-word after a report verb, especially 'why'.

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Other questions can also consist of only a very few words when the context makes it clear what is meant. Short questions of this kind are often used to express surprise or to offer something to someone.