Singing in Chinese!

This is one thing that that I can't get my head around - how do you sing in tones? Nicholas doesn't seem to have a problem with it and will often be heard singing along to his Chinese cds. There is one song that I know he knows all the words to - but he is reluctant to perform in front of the camera. We tried for about ten minutes, but at no point could I get a recording of him doing the whole song. He would sing it perfectly as soon as the camera went away, though! This is the final attempt - he gets half way through and walks off saying 'Mummy, turn off Chinese!'