Systems which work according to very simple rules, still produce very complex behaviour. Small changes in parameters can have huge consequences, for example water-molecules at 1°C or -1°C, although for an individual molecule the difference is very small (the average speed of the molecule drops about 0.4%) the behaviour of the systems as a whole is very different.

Computer offer the possibility to study the effect of parameters. Unfortunately the rules for normal systems can not be easily transformed to a computer simulation. Because computers are discrete, they are too slow. So normally only a very small part of a system can be simulated. Somehow a result for a small system has to to converted to a conclusion about a large system. Small changes can have huge consequences, this also applies to errors, so the errors (in converting the real system to a computer model, in approximations of the computation) can completely change the results.