This a small example that shows how to create bubbles in flash

var bubbleCount:Number = 30;
var mWidth:Number = Stage.width;
var mHeight:Number = Stage.height;
var i = 0;
var minSpeed:Number = 2;

for (var i=0;i<bubbleCount;i++) {
 var bubble = this.attachMovie("bubble", "bubble" + i, i);

 bubble._x = Math.random() * mWidth;
 bubble._y = Math.random() * mHeight;
 bubble._xscale =  bubble._yscale =  bubble._alpha = 40 + Math.random() * 60;

 bubble.yspeed = Math.random() * 3 + minSpeed;
 bubble.onEnterFrame = function ()
 {
 this._y = this._y - this.yspeed;
 if (this._y <= 0)
 {
 this._y = 400;
 this._x = 10 + Math.random() * mWidth;
 }
 if (this._x >= mWidth || this._x <= 0)
 {
 this._y = 400;
 this._x = 10 + Math.random() * mWidth;
 }
 };

}

This is sample application with above code

download source