Find a stagnant or slow moving body of water and collect a small amount of dirt from the bottom in one jar. Fill the jar the rest of the way with water from the body, being careful not to disturb the dirt more than necessary.
Use the other jar to collect algae and other tiny underwater plants. You may want to collect tiny animals like shrimp or snails. Be careful not to collect animals, however, that will devour the entire system.
Let the water in the first jar clear up before adding the algae and animals. Leave an inch of air space at the top of the jar and screw on the lid. Place the jar on a windowsill or another spot where it will get a good amount of sunlight. Observe and document the changes that happen in the jar over time.
Collect a small handful of pebbles and add them to the bottom of the jar. Collect a few small rocks with moss growing on them. Add those on top of the pebbles.
Gather sand and dirt from different areas. For best results, make sure the dirt you use comes from a wooded area with lots of bacteria from decomposing plants. Add moss on top of the sand.
Find a small plant such as a fern on an herb to place inside the jar to create oxygen. Add enough water from a pond or a stream to cover the bottom layer of pebbles. Cover the opening of the jar with the saran wrap and secure with a rubber band. Place the jar in a window. Observe and document the changes in the jar.