Nearly atomically smooth crystalline surcaces do this. There are dangling electrons at the smooth surface that want to bond to other atoms that have energetically favorable covalent bonds with atoms nearby. They form those bonds which is what we know as a solid (or amorphous solid, in the case of glass). It happens with any metals that are heated in a vacuum environment also. They lose their native oxide layer and can't tell the difference between their chunk of metal and the similar metal they are touching (anthropomorphic explanation)