Enya - Echoes In Rain

http://en.wikipedia.org/wiki/Emergence
One reason why emergent behaviour is hard to predict is that the number of interactions between components of a system increases combinatorially with the number of components, thus potentially allowing for many new and subtle types of behaviour to emerge. For example, the possible interactions between groups of molecules grows enormously with the number of molecules such that it is impossible for a computer to even list the arrangements for a system as small as 20 molecules.^{[citation needed]}