A sewage system removes waste products from a city. The excretory system removes waste products from the body. This analogy is quite common,' cause most people can understand it. But then again, most people have a day in day out understanding of the human excretory system also.
METAMIA is a free database of analogy and metaphor. Anyone can contribute or search. The subject matter can be anything. Science is popular, but poetry is encouraged. The goal is to integrate our fluid muses with the stark literalism of a relational database. Metamia is like a girdle for your muses, a cognitive girdle.