Strumming on Chords

I wonder how tunes are created initially. The first humans who invented them might be humming a repetitive melody and it successfully became a catchy tune. And now, look at the invention of human... innovation has come a long way to provide humans with sounds that pleases the ears, music

anyway have been practising "time of your life" by greenday.. such a wonderful song. simple yet meaningful. i still can't change the notes swiftly, at least not to the level of the actual strumming.. and i can't seem to sing and strum at the same time.. need practise