Cultures are externally affected via contact between societies, which may also produce—or inhibit—social shifts and changes in cultural practices. War or competition over resources may impact technological development or social dynamics. Additionally, cultural ideas may transfer from one society to another, through diffusion or acculturation. In diffusion, the form of something (though not necessarily its meaning) moves from one culture to another. For example, hamburgers, fast food in the United States, seemed exotic when introduced into China. "Stimulus diffusion" (the sharing of ideas) refers to an element of one culture leading to an invention or propagation in another. "Direct Borrowing" on the other hand tends to refer to technological or tangible diffusion from one culture to another. Diffusion of innovations theory presents a research-based model of why and when individuals and cultures adopt new ideas, practices, and products.