infinite sub-categories in MySQL

what I am trying to do is a forum where you can have a sub-forum that is a sub-forum of sub-forum... but as many as you want?
how do I desighn the schema in MySQL? any hint much appreciated

FrozenVoid, I thought you were leaving.

forum: (forum_id, parent_id, …)

`parent_id` being a nullable reference to another `forum_id`; only top level forums have a NULL `parent_id`

>>3
This isn't StackOverflow. Don't encourage the kiddies.