What part of speech is what?

[Always wondering]

What part of speech are the question words: what, how, when, who, where, why?

As in:
Why are we here?
Who will go with me?
When will it rain?
What time is it?
How will you know?

 

[philo2009]

Why are we here?
How will you know?
When will it rain?
- adverbs

Who will go with me?
- pronoun

What time is it?
-adjective (determiner)