Intermolecular forces such as Van der Waals forces, hydrogen bonds, and dipole–dipole interactions are typically not sufficiently strong to hold two apparently conformal rigid bodies together, since the forces drop off rapidly with distance, and the actual area in contact between the two bodies is small due to surface roughness and minor imperfections.
However, if the bodies are conformal to an accuracy of better than 10 angstroms (1 nanometer), then a sufficient surface area is in close enough contact for the intermolecular interactions to have an observable macroscopic effect—that is, the two objects stick together. Such a condition requires a high degree of accuracy and surface smoothness, which is typically found in optical components, such as prisms.