"Since plants produce their own food, they need not get it from an outside source. Under the right conditions, then, plants are self sustaining." Take a two liter bottle and cut the bottom off about 1/4-1/3 from the bottom. Fill the bottom part with soil. Plant seeds or replant seedlings or a plant in the soil. Moisten the soil well, but do not soak it. Take clear packing tape to reseal the top part to the bottom part. A self-contained biosphere.