If metals collides in space

On earth if two metals collide they will repel each other with a sound. We use special mechanisms if we want to attach two metals.

But on space it has some other effects. If two metals collide in space then will attach to each other, they will bond and will attach to each other permanently. Well this seems strange. on earth we do a process call welding to attach two metals but what happens in space is called cold welding.

The question comes in mind that why it happens?

The answer is because the atoms of the metals have no way of knowing that they are different pieces of metal so they join together.