How to make gnus prompt for password?

[Rodolfo Medina]

Hi, all.

In order to fetch mail from the pop server,
I put into my ~/.gnus.el the following lines:

(setq mail-sources '((pop :server "popmail.libero.it"
                          :user "romeomedina"
                          :password "my_password")))

and everything works fine. 
Now, however, I would like to avoid writing the password in the ~/.gnus.el,
and that the user was prompted for it,
as it is said in the section `Mail sources' of the Gnus Manual,
p. 140 of the latest 2003 version.
But when I try and cancel the above line

:password "mypassword"

, Gnus doesn't manage to fetch the mail any more.
Does anyone have any suggestion?

Thanks,
        Rodolfo