People often talk about top-down and bottom-up approaches when designing software.

The top-down approach starts with a large problem and breaks it down into a set of smaller problems. Then, if the problems are not small enough, as discussed when we looked at the Single Responsibility Principle, we further break down the problem into even smaller ones. The process repeats and eventually the problem is small enough to design and code.

The bottom-up approach works in the opposite direction. Given domain knowledge, you can start creating building blocks, and then create more complex ones by composing from these building blocks.

Regardless of how it is done, eventually there will be a set of components that work with each other, thereby forming the basis of the application. 

I like the metaphor. Even a 5-year old child can build a variety of structures using just several kinds of Lego block. Imagination is the limit. Do you ever wonder why it is so powerful? Well, if you recall, every Lego block has a standard set of connectors: one, two, four, six, eight, or more. Using these connectors, each block can plug into another block easily. When you create a new structure, you can combine it with other structures to create even larger, more complex structures.

When building applications, the key design principle is to create pluggable interfaces so every component can be reused easily.