Everyone knows that water is a molecule in which a single oxygen atom is linked to two hydrogen atoms - H20.
But the story of water gets a lot more scientifically interesting the deeper you go. Water actually exists in two different forms, called isomers, at the molecular level.
They have almost identical physical properties, you can't tell the difference, but chemists can tell them apart by the relative orientation of the nuclear spins of the two hydrogen atoms. They are called ortho-
depending on whether the spins are aligned in the same or opposite direction.