Thus far, we have examined the Earth in a rather static fashion by learning about places and regions, how maps are created, and how geographic information is gath- ered and analyzed. However, the Earth is dynamic and constantly changing, and one of the reasons places change is because of diffusion. Diffusion is the spread of ideas, objects, inventions, and other practices from place to place. As people migrate or move to a new area, they bring their ideas, objects, and the like with them in a process call relocation diffusion. Another diffusion process involves the spreadoutward from a core area that contains the idea, cultural practice, etc. This type of diffusion is expansion diffusion, and this type of diffusion can occur from person-to-person contact (as with a contagious disease) or through a hierarchy, or stratified condition, where the idea might originate in a major city, spread to medium-sized cities, and so on to smaller cities (Figure 1.13). In Chapter 6, we will examine how religion has spread across the world through both relocation and expansion diffusion, and in Chapter 10, we will see that domesticated plants and animals have diffused extensively across the Earth.