Why is oxygen essential for life?, It changes color, It produces energy, It makes water sweet, It creates rocks, Why can oxygen be harmful in large amounts?, It damages cells, It stops breathing, It removes water, It freezes cells, What was the Great Oxidation Event?, Plants disappeared, Oceans formed, First animals appeared, First oxygen buildup, How do plants produce oxygen?, Evaporation, Digestion, Photosynthesis, Respiration, Why were ancient insects larger?, Colder climate, Higher oxygen levels, Fewer plants, More food, When did animals first appear?, 70 million years ago, 700 million years ago, 7 billion years ago, 7 thousand years ago, Why is sea algae important?, It warms oceans, It feeds insects, It cleans rocks, It produces most oxygen.

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