Often, the cavity is large enough to accommodate only a single small gas molecule. Unlike metal- and covalent- organic frameworks, which are extended networks of molecular building blocks held together by directional coordination and covalent bonds, respectively, the assembly/packing of porous organic cage molecules to form a bulk porous cage solid is dictated by the geometry of the molecules and non-covalent/non-coordination interactions between them. On the order of hundreds of porous organic cages have been reported. For deployment in molecular separations or vapor sensing, the size and shape of the cavities in a porous material can strongly influence the adsorptive selectivity. In a shapeselective molecular separation, the shape of the cavity or window in a porous material is such that it accommodates a subset of molecular species but excludes the remaining species through steric hindrance. Aside from geometric exclusion, the size and shape of the cavity influence the enthalpy of adsorption, e.g. by encompassing the adsorbed gas molecule with chemical moieties in close proximity with which to interact, and the entropy of adsorption, e.g. by minimizing the loss of rotational entropy in the cavity. Therefore, it is important to † The energy requirement for separating a mixture has a thermodynamic limit, 3 of course.