text: Kinda depends on how patient you are and what solids you are talking about.  \n\nVery few substances have a surface which has not in some way interacted with the environment to form a different substance. For example, aluminum very quickly oxidizes in air to form aluminum oxide which is much different from pure aluminum.  If you take two very clean surfaces in a near vacuum, two metals surfaces can come so close together that they actually can swap electrons and weld together.  In normal earth environments, only serious pressure forcing the two together can get this kind of interaction.  \n\nIn all substances above 0 K, the motion of the particles make it possible for individual molecules/atoms to move enough to "jump the gap" if they are warm enough, close enough and have small enough differences between the forces that keep them together versus those that attract them to the other team.