Self Join Help

Im trying to figure out Self Joins with PostgreSQL.  The output of the
second SQL is correct, because of the where a.id = b.pid,
but I would like to return all rows that are part of the tree.
i.e.
Foo
Apache
 - PHP
XHTML
News
 - World News
 - Tech News

Any help would be appreciated.
Thanks.

test=# select * from topics;
        id        |       pid        | topicname
------------------+------------------+------------
 AFAdDFoAPNX6wKbr | 0                | Foo
 AFAdDFoAPgTi9tAE | 0                | Apache
 AFAdDFoAPgTjCa4V | AFAdDFoAPgTi9tAE | PHP
 AFAdDFoAPlv1ENRn | 0                | XHTML
 AFAdDFoAPoSEWZaq | 0                | News
 AFAdDFoAPoSEaRPV | AFAdDFoAPoSEWZaq | World News
 AFAdDFoAPoSEee5_ | AFAdDFoAPoSEWZaq | Tech News
(7 rows)

test=# select a.topicname as parent, b.topicname as child from topics as
a, topics as b where a.id = b.pid;
 parent |   child
--------+------------
 Apache | PHP
 News   | Tech News
 News   | World News
(3 rows)