This document discusses natural cycles including the oxygen, carbon, water, and nitrogen cycles. It begins with introducing the objectives and importance of understanding these cycles. It then covers the oxygen cycle in detail, explaining that oxygen cycles through various forms as it is used in respiration and photosynthesis by plants and animals, and is also present in compounds like carbon dioxide and water. The carbon cycle is similarly explained, noting that carbon dioxide levels in the atmosphere are increased by human activities like burning fossil fuels.