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Cold Welding
If two clean, flat metals are brought into contact in vacuum, they will stick together as one. This is called cold welding.
Richard Feynman explained it as:
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.
โโRichard Feynman, The Feynman Lectures on Physics, 12โ5 Friction