I was wondering this question the other day and have read a few websites but a lot of it goes over my head. I have two questions, can anyone explain in relatively simple terms? I am happy to read the more detailed science behind the concepts, but need the concepts to be stated pretty plainly. What makes a solid, solid? Why, if I place one solid on another, do they not merge? In other words, what maintains the individual identity of two blocks of lead even when placed together? A fluid of the same kind of course does merge.