A stream. Fresh water bubbles up through the ground or down from the mountain. It’s clear, and fresh, and sweet.
No matter what you do to the water, it remains clear, and fresh, and sweet.
You can throw mud, or rocks, or waste, or filth into it, and the water will carry it away, and the stream will remain clear, and fresh, and sweet. You can change its path, but still it flows.
A well. Fresh water bubbles up through the ground, or is poured in from above. It’s clear, and fresh, and sweet. Better yet, a well isn’t as fickle as the stream: it doesn’t swell and shrink with the seasons. A well also serves as a place to store as much clear, fresh, sweet water as you’d like.
But a well is fragile. Throw anything in it and the water spoils. If there is waste or filth nearby, the well is soon filled with waste and filth. The well is also stationary, its brittle walls susceptible to even minor pressures.
It’s much better to be the stream.