Crack the Code: Educational Crossword Fun

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Across
  1. 2. The layer of air around Earth. (9 letters)
  2. 5. The natural satellite of Earth. (4 letters)
  3. 7. The body system responsible for movement. (8 letters)
  4. 8. The fourth planet from the Sun. (4 letters)
  5. 9. The planet known as the "Red Planet." (4 letters)
  6. 11. The smallest unit of life. (4 letters)
  7. 13. The study of rocks and Earth's structure. (7 letters)
  8. 14. The largest ocean on Earth. (7 letters)
  9. 16. The freezing point of water in Celsius. (5 letters)
  10. 17. The chemical symbol for gold. (2 letters)
  11. 19. The smallest continent by area. (9 letters)
  12. 20. A famous physicist who developed the theory of relativity. (8 letters)
Down
  1. 1. A polygon with three sides. (8 letters)
  2. 3. The gas we breathe in to survive. (6 letters)
  3. 4. A shape with no corners or edges. (5 letters)
  4. 6. The green pigment in plants. (11 letters)
  5. 10. The process by which water turns into vapor. (11 letters)
  6. 12. The first ten amendments to the U.S. Constitution. (4 letters)
  7. 15. The largest desert in the world. (6 letters)
  8. 18. The study of plants. (8 letters)