Grammar - fragments
A fragment is a word or a phrase which stands by itself but which does not make up a complete sentence. Fragments are very common in ordinary speech, in advertisements and even in newspapers. They may be used very sparingly in formal writing; when used, they should be followed by a full stop, a question mark or an exclamation mark, as appropriate:
Will the Star Wars project ever be resumed? Probably not.
We need to encourage investment in manufacturing. But how?
Can England beat Australia? Absolutely!
The judicious use of fragments can add vividness to your writing, and they are quite acceptable in writing which is somewhat informal. But don't overdo them: if you use too many fragments, your work will become breathless and disjointed.