Bone growth as a metaphor for work in Wikipedia.

In the human body, there are two groups of cells that manage the production and refinement of bone. Osteoblasts create new bone while osteoclasts break bone down. These cells are constantly working in parallel to manage our bone structure and repair damages. When a fracture takes place, osteoblasts come in to calcify the tissue surrounding the break. They aren't very picky about what or where they calcify so you'll end up with a large mound of bone where the break was. Over time the osteoclasts will trim and refine this mound down until the bone reacquires its original/desired shape.

This is an excellent metaphor for how articles are written in Wikipedia. There is a very large group of editors who do not make many edits on an individual basis, but they contribute the vast majority of content that makes it into the encyclopedia. They behave like osteoblasts in that they contribute large amounts of material but they don't have the experience to know what sort of content is appropriate or how to format their contributions. A smaller group of more active members of the encyclopedia (Wikipedians) perform the role of osteoclasts by trimming unencyclopedic content and refining what is left into coherent articles.

In order for a human to have a healthy skeletal structure, a balance between bone formation and bone trimming has to be maintained. In the same way, the balance between content contributors and content refiners in Wikipedia must be maintained.