Book 30, Question 137
Can a jar of pond water take care of itself?
A sealed jar with pond water, some plants, and tiny animals can stay alive for a while as a miniature world.
Plants use light to make food and oxygen, tiny animals use the oxygen and make carbon dioxide, and the plants use that. It is a model of how a lake's cycles work. Over time the balance tips, so it may need adjusting. Ask a grown-up before starting one.