[Zope] - What constitutes a transaction?

I'm obviosuly missing something here. I have a product, a subclass of Folder. It has several methods that do things to its internal state. Some of these methods cause a transacton to be generated (Ie, when adding or deleting objects, setting properties, etc., the method shows up in Undo). When I shut down zope and restart it, these changes are still there. My added methods have effect that is forgotten at shutdown, and they don't show up in the Undo list. I have another Product where my added methods do work correctly. What am I missing?