system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it that when you touch two pieces of, say, iron, together they don't "stick like it does to the rest of "itself"
gpt
Because the two pieces will have a layer of oxides, coatings, oils, dirt or other impurities on their surface. In addition, the surfaces will be rough, so the actual contact area will be miniscule.

But do away with that, and metal will 'cold weld'. For instance, make a moving joint on a spacecraft in the vacuum of space. The two pieces will wear, exposing fresh, uncontaminated metal, and will fit exactly, making the contact area large. It will not rust, because there is no oxygen. The two faces of the joint will cold weld, becoming one.