How to randomise selection from library?

could anyone tell me how to make the following peice of code select randomly from various movieclips in the library, rather than using just one movieclip?

depth = 0;
function makeNewClip(){
newClip = _root.attachMovie(‘drop6’, ‘drop6’+depth, depth++);
newClip._x = Math.random()*Stage.width;
newClip._y = -50;
newClip.speed = (Math.random()*10);
newClip.onEnterFrame = function() {
this._y+= this.speed;
};
}
makeNewClip();
setInterval(makeNewClip, 2000);

:insomniac: