In the mechanical world, complex storage systems, by necessity, use different mechanical form factors at each level: In a file cabinet, you never see folders inside folders or file drawers inside file drawers.
Most mechanical storage systems are simple, composed either of a single sequence of stored objects (like a bookshelf) or a series of sequences, one level deep (like a file cabinet).
Take the example of the ubiquitous file folder in modern computer operating systems. As a mechanism for organizing documents, it is quite easy to learn and understand because of its similarity to a physical file folder in a file cabinet.
