- If you are using a core theme such as Garland, you need to create your own theme. Even if you're only modifying the theme for the purpose of this tutorial (and of course, to get collapsible blocks). It's always recommended that you do not overwrite any sort of core Drupal files.

- This is untested in Drupal 5

Want to create collapsible blocks in Drupal 6 that actually remember the state the user left them in?

It's pretty easy with JQuery which is built-in to Drupal 6.

Tutorials on how to extend usability using JQuery and Drupal