Suppose that we bring two systems A and B, together to form a combined system C. There is an interface where A and B are in contact. Near this interface, particles of A interact with those of B. However, since A and B are macroscopic, the number of particles in the neighborhood of the interface is extremely small compared to the number in the bulk of A or B. Thus, on short time scales, we can ignore effects of the interface (on long time scales, the cumulative effect of small interface effects is important).