 Let's make our own mini ecosystem. First, get a jar and pour a layer of gravel and sand. Then add a layer of activated charcoal and finish up with a layer of houseplant potting soil. With tweezers, add your plants. Here we have our ficus pumila, water just a little bit and close your jar. The water will evaporate, condense and precipitate making a self-sustained ecosystem in your jar. And that's how you make a closed terrarium.