A picture of a tree, drawn using fractal or mathematical methods. It is relatively easy to write a computer program to draw a tree when one realizes how self similar a tree is. The twigs on trees look like the branches on which they grow, and the branches look like the trunk. One could write a function "Branch" which simply draws one branch, and then calls copies of itself to make sub-branches on that branch.

In C'ish pseudo-code, a simple 2d example:
--------------


#include "math.h" // for the trig stuff and random number generator

#include ??? // whatever you need to do graphics on your platform

void Branch(int x, int y, float len, float angle)
// x,y are the starting coords on screen,

// len is the branch length in pixels,

// and angle is the angle of the branch in radians.

{

   int color,x2,y2; // branch color and endpoint..

   float childlen; // the length of any children off this branch..

   float childangle; // the offshoot angles of any children


  if(len>3) // if its a long branch, make it brown.

     color=BROWN;

  else

     color=GREEN; // otherwise make it green.

  // draw the 'branches' and 'leaves' as a simple lines.
  // Get more fancy than this, please.

   x2=x+len*cos(angle);
   y2=y-len*sin(angle);

   DrawLine(x,y,x2,y2,color); // draw branch

  if(len>3) // if this branch is long enough to have children...
   {

     childlength=len*(float)(rand()%80+10)/100.0;

     childangle =3.14159/2.0*(float)(rand()%100)/100.0;

     Branch(x2,y2,childlength,angle-childangle);
     Branch(x2,y2,childlength,angle+childangle);

   }

}

void main() // time to plant our tree
{

   // set up your graphics here...

   // ...648x480 pixels, for example..

   Branch(320,480,100,3.14159/2.0); // and get to it!

  Wait_For_Keypress(); // all done.

}