Introduction: What is a food chain?
Together with students, try to figure out what food chains are.
Activities in this lesson
Discussion: How organisms purify water
Discuss with students how water organisms purify water from organic pollution. (Organic pollution from homes and farms represents food for some macroinvertebrates and bacteria that use it as food and decompose it in inorganic matter, which is used by plants and algae for their growth. Water purifies itself through including organic pollution in food chains in the biological cycle.)