pore

But in this case, fitting into a given pore depends on many more characteristics than just the shape of the toddler's peg, allowing certain pores to select for some substances over others -- what chemists call the "selectivity" of "molecular encapsulation," or just selective encapsulation.

The interior of this ring -- in general -- forms the cavity or pore where only certain substances "fit."

On top of this, there are types of macrocyclic molecular crystals where the pore only appears in the presence of certain conditions such as heat or pressure or that of other, "guest" molecules.

The rest of the time, there is no pore.

This now-you-see-it-now-you-don't type of cavity is called a "latent pore."