How do you create an empty tar file?

[A.V. Raman]

I have to create an empty tar file that can later be appended to.
I tried

$ tar cf foo

Tar did create foo.  But all attempts to append further files to it
seem to fail, for although the files do go into foo, I'm not able
to list them with tar vtf foo nor extract them with tar xf foo.

Thanks very much for any help.

- & (Anand)