Imagine a girl at the beach idly playing with dry sand. Handful by handful she trickles grains between her fingers. At first the sand is flat and the grains lie close to where they fall. Each grain's motion can be understood in terms of its individual physical properties. The pile grows and its sides get steeper. Soon she will see little sand slides. The higher the pile gets, the bigger the slides. Eventually, the slope of the sides cannot be increased, however much more sand is added. At the same time the avalanches get so large as sometimes to span all or most of the pile. At that point, the system is far out of whack. Its behaviour can no longer be understood in terms of the behaviour of individual grains. The avalanches have a dynamic of their own which can only be understood from a description of the properties of the whole heap, not from a description of single grains. The sandpile has become a complex system.