Flash AS3 how to make movieclips start at different times?

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So i have been programming a game and i wanted my images to start at different times and i have this coding:
for (var i:int = 0; i < 25; i++) {
var r:SnowFlake = new SnowFlake();
var randomvalue:Number = Math.random();


r.x = -50 + Math.random()*960;
r.y = -50 + Math.random()*200;

r.scaleX = r.scaleY = Math.random()*3;
r.alpha = 1;

this.addChild(r);
}
how do i get them to start at different times?