Wash out a 2-liter soda bottle, and allow it to dry for one hour.
Cut the bottle in half around the bottle's circumference using a box cutter. Place the top half of the bottle aside.
Fill the bottom half of the bottle with potting soil, and moisten it with 1 cup of distilled water.
Dig three or four 2-inch holes, and place a small tropical plants into each hole. Cover the base of each plant with more potting soil. Press the soil down slightly.
Lay moss down on top of the soil. Moss does not need to be planted and will thrive in a very humid environment.
Place the top back onto the bottom of the bottle, and run your finger around the seam to help the top edge overlap the bottom edge. Push down the top of the bottle until it has overlapped the bottom by 1 inch.
Place the soda bottle ecosystem in an area of your home that is warm. Make sure the bottle is undisturbed as much as possible.