A complex system is one in which many little actors, each one simple as can be, interact with their neighbors and give rise to a rich new level of behavior. The actors — agents is the usual term — might be cells interacting in an organism or creatures in an ecosystem. They might be people bartering and selling and unwittingly generating the meteorological gyrations of the economy. The agents need not be living things. Put together hordes of molecules, each made from two hydrogen and one oxygen atom, and you get a mysterious quality called wetness. There is nothing about a single molecule of H2O that is wet. Wetness, in the lingo, is an emergent phenomenon.