Which vitamin is primarily synthesized in human skin upon sunlight exposure?

Which common household chemical reaction produces a foaming effect often used in classroom volcano experiments?

Which gas makes up approximately 21 percent of Earth's atmosphere and is essential for respiration?

Which common antibiotic was first discovered from a mold by Alexander Fleming?

Which everyday phenomenon demonstrates the principle of capillary action in plants?

Which everyday observation demonstrates the concept of inertia in physics?