Nitrogen fixation—by bacteria or lightning—converts atmospheric N₂ into biologically usable ammonia or nitrates.

The Opium Wars (1839–42, 1856–60) were fought between China and Britain, largely over the opium trade and sovereignty.

Marie Curie, with Pierre, discovered radium and polonium in 1898, pioneering radioactivity research and earning two Nobel Prizes.

The 1929 crash triggered the Great Depression, causing widespread unemployment, poverty, and global economic collapse.

Aristotle tutored Alexander, influencing his leadership and spreading Greek philosophy across conquered regions of the ancient world.