Water, the medium of life

The big idea

Life began in water, and every cell is still mostly water. That isn't luck. Water's unusual behaviour comes from one feature: its molecules are polar — slightly negative at one end and slightly positive at the other.

Explained simply

A water molecule ($\text{H}_2\text{O}$) is one oxygen atom bonded to two hydrogen atoms in a bent shape. Oxygen pulls the shared electrons towards itself more strongly than hydrogen does. So the oxygen end carries a small negative charge ($\delta^-$) and the hydrogen ends carry small positive charges ($\delta^+$). The Greek letter delta just means "a little bit of".