Why 誰か and not 誰?


I thought that 誰 meant 'who', such as in:

> 彼は誰ですか。'Who is he?'

Then, why in this sentence it is 誰か ('someone') which is used and not 誰?

> 彼女が誰か知っていますか。'Do you know who she is ?'




In this context, you can think of `誰か` as a contraction of `誰なのか` which stands
for `who it is`.

