Computers, unlike humans, can quickly follow a set of rules (a program) to give solutions to problems. This ability gives science an amazing power it did not have before the 20th century. The computer, furthermore, allows study of topics that before became too quickly complex to be worthwhile, such as systems that are created from simple rules applied over many iterations. These complex systems are not only interesting for their mathematical beauty, like fractals, but also because they can be applied to natural systems. Cells, insects, forests, populations, and the brain can all be modeled with computers to help us better understand how they work. This work has implications in artificial intelligence, as well as modeling systems which focuses on working out how a system works as opposed to simply replicating its function.