Word association can link just about any two common (root) words in the English language using an average of three steps (degrees of separation), says a team of scientists at Arizona State University.
The researchers think the network structure of a language probably has its origins in the nature of cognition and memory. Different concepts, such as “actor” and “universe,” are closely linked by a short series of semantic steps. This, say the researchers, makes it easier to carry out mental searches when using language.

Motter, A. E., de Moura, A. P. S., Lai, Y.-C. & Dasgupta, P. Topology of the conceptual network of language. Physical Review E, 65, 065102, (2002).