In space, two pieces of metal can weld together with no heat or other substances present!
Normally if you want to join two pieces of metal together, you use heat, or some sort of chemical process to do it. However, when metal is in a vacuum (like in space), two different pieces of metal will weld together with no other external force present.
It's called Cold Welding and it was discovered in the 1940s. And it might sound very impracticable, but it's become extremely useful in nano construction, tiny parts can be joined without the bulk of adhesives or melting.