What is the coolest thing you can do in <10 lines of simple code? Help me inspire beginners!

Enter this code in your address bar (in your browser) and press enter. Then you can edit all the content of the webpage!

javascript:document.body.contentEditable='true'; document.designMode='on'; void 0

That is the coolest "one-liner" I know =)

When I first wrote this.

10 PRINT "What is your name?"
20 INPUT A$
30 PRINT "Hello " A$
40 GOTO 30

It blew people away! The computer remembered their name!

EDIT: Just to add to this. If you can convince a new programmer this is the coolest thing they can do, they will become the good programmers. These days, you can do almost anything you want with one line of code to run a library somebody else wrote. I personally get absolutely no satisfaction from doing that and see little benefit in teaching it.

PHP - the Sierpinski gasket a.k.a the Triforce

OK, it's 15 lines of code but the result is awesome! That's the kind of stuff that made me freak out when I was a child. This is from the PHP manual:

$x = 200;
$y = 200;

$gd = imagecreatetruecolor($x, $y);
 
$corners[0] = array('x' => 100, 'y' =>  10);
$corners[1] = array('x' =>   0, 'y' => 190);
$corners[2] = array('x' => 200, 'y' => 190);

$red = imagecolorallocate($gd, 255, 0, 0); 

for ($i = 0; $i < 100000; $i++) {
  imagesetpixel($gd, round($x),round($y), $red);
  $a = rand(0, 2);
  $x = ($x + $corners[$a]['x']) / 2;
  $y = ($y + $corners[$a]['y']) / 2;
}
 
header('Content-Type: image/png');
imagepng($gd);