http://shnitzelkiller.deviantart.com...stem-108256000
I just learned how to use classes to control the behavior of multiple instances, to create effects such as brownian motion and smoke and stuff. It's really boring to learn, but you can use it for all kinds of stuff. In this case, it simulates welding, with sparks, fire, and smoke. Although the last two kind of revert into each other, you get the idea. The caustics thing I made is still better, but in this is just an effect, so it's not really constructive. It could be used for something similar, though.

it lags if you hold the left mouse button. how is it cool?

S' fun for a little bit. I assume this was your first go, so it's good not to be too ambitious.

That's pretty cool.

@Neosano: L2lower quality.

It lags on the web, but it's very fast as a swf file. Anyway, it's sort of a test. The test goes like this:
package
{
import flash.display.MovieClip;
import flash.events.*;

public class Particle extends MovieClip
{
private var windX:Number = 0;
private var windY:Number = 0;
private var brownianMotion:Number = 1;
private var sq:Number;
private var maxSpeed:Number = 4;
private var growthFac:Number = 0.18;
private var kill:Boolean = false;

public function Particle()
{
addEventListener(Event.ENTER_FRAME,loop);
this.scaleX = 0;
this.scaleY = 0;
}
private function death()
{
if (kill)
{
removeEventListener(Event.ENTER_FRAME,loop);
stage.removeChild(this);
}
}
private function loop(e:Event):void
{
if (growthFac > 0)
{
growthFac -= 0.01;
this.scaleX += growthFac;
this.scaleY += growthFac;
}
windX+=(Math.random()-0.5)*brownianMotion;
windY+=(Math.random()-0.5)*brownianMotion;
sq = windX*windX + windY*windY;
if (sq > maxSpeed)
{
windX = windX/Math.sqrt(sq)*Math.sqrt(maxSpeed);
windY = windY/Math.sqrt(sq)*Math.sqrt(maxSpeed);
}


this.y += windY;
this.x += windX;

windY -= 0.1;

if (this.x > stage.stageWidth)
{
windX *= -1;
}
if (this.x < 0)
{
windX *= -1;
}
if (this.y > stage.stageHeight)
{
windY *= -1;
}
if (this.y < 0)
{
windY *= -1;
}
this.alpha -= 0.005;


if (this.alpha <= 0)
{
kill = true;
death();
}
}
}
}

and this

package
{
import flash.display.MovieClip;
import flash.events.*;

public class Spark extends MovieClip
{
private var gravity:Number = -1;
private var acX:Number = 0;
private var acY:Number = -7;

private var bounced:Boolean = false;

private var prevX:Number;
private var prevY:Number;

private var currX:Number;
private var currY:Number;

private var speedX:Number;
private var speedY:Number;

private var rotVal:Number;

public function Spark()
{
acX += (Math.random()-0.5)*10;
addEventListener(Event.ENTER_FRAME, loop);
}
private function loop(e:Event):void
{
acY -= gravity;


prevX = this.x;
prevY = this.y;

this.y += acY;
this.x += acX;

currX = this.x;
currY = this.y;

speedX = currX - prevX;
speedY = currY - prevY;

rotVal = Math.atan2(speedY,speedX)*180/Math.PI;
this.rotation = rotVal;
if (this.y > stage.stageHeight)
{
acY*=-1;
bounced = true;
}
if (bounced)
{
if (acY < 1)
{
this.alpha -= 0.1;
if (this.alpha <= 0)
{
removeEventListener(Event.ENTER_FRAME, loop);
stage.removeChild(this);
}
}
}
}
}
}