the polarity of water
Hold a tiny magnet and you can feel one end pull and the other push. A water molecule has something like that built in: it has a slightly negative end and a slightly positive end, even though as a whole it carries no charge. This lopsidedness is called polarity, and it is the single property from which almost all of water's life-giving behavior springs.
A water molecule is one oxygen atom bonded to two hydrogen atoms (H2O), arranged in a wide V shape rather than a straight line. Oxygen is greedy for electrons, so it pulls the shared electrons closer to itself, leaving the oxygen end slightly negative and the two hydrogen ends slightly positive. Because the molecule is bent, these pulls do not cancel out — one side really is more negative than the other. The result is a little electrical dipole: a molecule with a plus end and a minus end. Opposite charges attract, so the positive hydrogens of one water reach toward the negative oxygens of its neighbors, knitting water into a constantly shifting, loosely linked network.
Polarity is why water dissolves salts (its charged ends surround and separate the ions), why it forms hydrogen bonds, and why it pushes oily molecules together (the hydrophobic effect). In molecular biology, polarity is the starting fact: it explains why some parts of a protein face outward into water and others hide inside, and why DNA's charged backbone sits comfortably in the cell while its bases tuck away. Almost every later idea in this field traces back to this bent, lopsided little molecule.
Bring a charged comb near a thin stream of water from a tap and the stream bends toward it — a kitchen demonstration that the water molecules really do carry a plus and a minus end that respond to charge.
A bent molecule with a plus end and a minus end.
Polarity does not mean water is charged overall — it is neutral. The plus and minus are tiny partial charges (often written with the Greek letter delta), arising only because the shared electrons sit unevenly.