Cranes (not the various large wading birds of the family Gruidae)

On my way to and from work I pass by a construction site where a medium sized crane is being utilized. I want you to think of the last time you saw a crane. How big was it? Have you ever seen a crane being built? If not, the situation can lead to the thought "how did it get there?" To give a general answer, I will assume that we all believe people constructed and erected the crane. As well, I assume we would all likely understand the steps to build a crane if someone cared to explain it in a patient manner. So what we have here is a product of push-pull causation, but the end result is quite amazing. Additionally, you have probably realized, or seen, that cranes can be used to create larger cranes.
The same can be said about computers. I think most people are impressed by computers and everyone thinks they work by electricity moving around, even if they cannot give specifics. Computers have been wonderfully useful as a tool to help make newer computers. I think it is also interesting that initially the crane or the computer was the end goal of production; its use was intended for tasks other than those involved in the replication of different versions of itself. Things sure have changed.

As you sit at your computer reading this, try to realize the complexity of how the computer works, but also the simplicity of what are ultimately the instructions to make said computer operate. Very simple things can lead to great complexity, and sometimes to things that were not specifically in the development specifications.
Can you think of any other examples of simple, small processes that lead to complex, large entities?