If two pieces of the same type of metal touch in space, they will bond and be permanently stuck together. This effect is called cold welding.

There are 3 kinds of bonds - Covalent, Ionic and metallic.

In metallic bonds, metal atoms are kept close to each other so electrons can freely move between them. The attraction between these electrons and all of the metal nuclei keeps them held together. Metallic bonds are also responsible for the conductivity of metals.

On earth, metals are almost always covered by a very thin film of oxide making metal-to-metal contact rare. In space there is no atmosphere to provide oxygen, which would renew the oxide coating. When the oxide wears off and metals are pressed together, there is true metal-to-metal contact. Since the metal surfaces are not microscopically smooth, the pressure is all taken out on a small number of microscopic high spots and the metals simply unite at these points.

You can try a similar effect in your own kitchen. Take a couple of ice cubes, let them warm up a bit, then press them together. They will weld into a single lump. Water already has all the oxygen in its molecule that it can accept, so it cannot oxidise further.

The effect has a lot of implications for spacecraft construction and the future of metal-based construction in vacuums.