One important property of a surface is its ability to interact with other atoms and molecules. In the study of surfaces, it might be important to have a large surface area. The large surface area can be particularly important in the case of a property that scales with the amount of surface area, such as the enthalpy change associated with adsorption. However, the surface must be regular for measurements to be meaningful. A very important surface, both for its homogeneity and its rather large surface area, is that of exfoliated graphite. The Gibbs energy of a surface is always higher than that of the bulk material because the surface atoms are not able to achieve bonds in all directions as the interior atoms can. In this sense surfaces are unstable relative to the bulk material. One consequence is that if there is flexibility within the surface, it will choose to have the minimum surface area.