There are problems that grow very fast. One such problem is known as the Travelling salesman problem. A salesman needs to take a tour of a certain number of cities. Each city should only be visited once, the distance (or cost) of the travelling should be minimal, and the salesman should end up where he started. This problem has exponential complexity. There are n factorial possibilities to consider. Adding one city (from n to n+1) will multiply the number of possibilities by (n+1). Most of the interesting problems have this complexity.