1. What vitamin do humans mainly get from sunlight?

2. How many teeth does a typical adult human have?

3. What part of the human eye controls the amount of light entering?

4. What is the hardest substance produced by the human body?

5. Why do identical twins have different fingerprints?

6. Which protein in red blood cells uses iron to transport oxygen throughout the body?

7. Which enzyme in saliva is responsible for beginning the digestion of starches in the mouth?

8. What is the name of the phase in the cardiac cycle when the heart muscle relaxes and the chambers fill with blood?

9. Which muscle in the human body is considered the strongest based on its weight and ability to exert force?

10. What is the name of the structural protein that is the primary component of human hair and nails?

11. What is the name of the substance that prevents the alveoli from collapsing during exhalation?

12. Which gland is often referred to as the 'master gland' of the endocrine system?

13. What is the primary function of melanin in the human skin?

14. What specialized structures connect cardiac muscle cells and allow them to contract in sync?

15. What percentage of human blood is typically composed of plasma?

16. What are the microscopic hair-like structures that move mucus out of the human respiratory tract called?

17. Which mineral is essential for the thyroid gland to produce its metabolism-regulating hormones?

18. What is the primary purpose of the blood-aqueous barrier in the human eye?

19. Why is the blood-testis barrier necessary for male fertility?

20. What two gases are exchanged across the blood-air barrier in the lungs?