When you cut something in half, why can't you simply put it back together?

When you cut something in half, why can't you simply put it back together? What once held it together that no longer can? Is it atoms binding together or something? And if that's the case, why don't they "bind" back together? 


The ELI5 method is that by separating the thing a microscopic barrier forms between the two, allowing them to "know" they are separate objects. For example, if you cut a piece of metal, the exposed surfaces oxidize extremely rapidly. This oxide layer prevents the two parts from reintegrating as that layer separates them.

There is an interesting phenomenon called Cold Welding where two parts can weld together without being heated. This occurs in space with extremely clean parts and is basically exactly as you say. As Feynman put it:

"The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to “know” that they are in different pieces of copper. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms “know” when they are not on the same part."

As well, one of the reasons for flux in soldering/brazing/welding operations is to clean off contaminants and oxide layers, allowing the metals to melted together and joined. With no flux you may not be able to penetrate the oxide layer and you will end up with remaining distinct interface and a failed joint.

 - ELI5: When you cut something in half, why can't you simply put it back together? What once held it together that no longer can? : explainlikeimfive