system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't I cut a solid, and then stick the two pieces back together again?

If I take a solid iron rod, and saw it in half, what is actually happening on an atomic level?  And why I can't I stick the two solid pieces back together, unless I convert one of the pieces to liquid first?
gpt
You can. In fact, it's known as cold welding and commonly used in space. 

Under normal conditions, the moment you cut it, contaminants stick to the surface and the metal reacts with the atmosphere to form an oxide layer. This prevents the previous metallic bonds from reforming. 

For other objects, they tend to have crystalline structures which have to be perfectly aligned to reform the bonds.