Source:
http://en.wikipedia.org/wiki/File:Rin_gong_at_Kiyomizu-dera,_Kyoto.JPG
A singing bowl can be created from any kind of hollow vessel. The rim is stimulated (such as by rubbing, or gently hitting) which causes the whole body to vibrate, serving as resonator of the air inside the vessel. Other instruments working on the same principle are drinking glasses and the glass harmonica, or handbell.
Source:
http://en.wikipedia.org/wiki/File:Rin_gong_at_Kiyomizu-dera,_Kyoto.JPG
http://www.youtube.com/watch?v=Bpw53tN6h8E