Think about a Bill of Material (BOM) application in manufacturing.
I have a motor. It contains, among other things a block, a head assembly, 4 pistons, various bolts, and a gasket kit.
The gasket kit contains 15 gaskets of various types.
The head assembly contains valves, a valve spring kit, bolts, camshaft, bolts, etc.
the valve spring kit contains 8 valve springs.
You have a heirarchial relationship.
Each record would contain information about the item in question, including a pointer (Foreign Key) to it's parent record...
If a part is at the top of it's heirarchy, then it has no parent, and IT'S parent record FK is null.
"Lisa, in this house, we obey the laws of thermodynamics!" - Homer Simpson
"I have my standards. They may be low, but I have them!" - Bette Middler
"It's a book about a Spanish guy named Manual. You should read it." - Dilbert