Databases or more correctly how Databases model things in the real world fascinate me.
A really good primer on why database tables use a single table to represent Hierarchical Data.
The two examples that that come to my mind are a Bill of Lading and a Recipe or Formula:
Bill of Lading is a parent, The items on the bill of lading are children
A Formula or Recipe is the parent, The ingredients are children, which in turn can have children of their own.
The Managing Hierarchical Data in MySQL article linked to above helps to get your head around how to structure the data sensibly in a database.
I give it 5 stars Dave.