Figure 1.

Depending on the relative sizes and shapes of a guest (red) and a host (blue), the guest mobility can be constrained. If the guest both can be placed inside the host cavity and cannot escape, it is caged (b); otherwise it is not caged (a, c). More precisely, (a) the guest might be too small and enter/exit the cavity through the host windows; (b) the guest might match the size/shape of the host cavity and be “caged”; or (c) the guest might be too big to fit within the cavity. In this example, we do not discuss how these pairs are formed.