Researchers have taken a major step forward in making previously undrinkable water drinkable, therefore tackling one of the biggest challenges faced by the planet -- some 1.2 billion people lack access to clean drinking water (a number which is set to grow as populations increase). By modifying graphene oxide membranes, the international team of researchers has created what is essentially a molecular "sieve". The selectively permeable membrane lets some molecules through while trapping others behind, producing water at various levels of cleanliness suitable for drinking or for industrial applications.