Water

The strangest ordinary substance and the precondition of nearly everything — a simple molecule of two hydrogen atoms and one oxygen whose freak properties make life and Earth's climate possible. Water filled the oceans, drove the cycle of rain, cradled the first cities on great rivers, carried history's deadliest plagues until people learned to clean it, and remains the resource humanity can least live without and most fiercely fights over.

A molecule of two hydrogen atoms and one oxygen should be unremarkable. Instead water breaks the rules. Almost every liquid shrinks and sinks as it freezes; water expands, so ice floats and lakes never freeze solid from the bottom up, which is the only reason fish survive winter. It dissolves more substances than any other common liquid, so it became the medium all life's chemistry runs in. It soaks up and carries vast amounts of heat, so it steadies the whole planet's climate. Every living cell is mostly water, and no life we know of can exist without it. This is the ordinary miracle that built the first cities, carried the worst plagues, and that nothing alive can do without.