function changeBkg(){

//create array to hold the class names
var classList=new Array();
classList[0]="b1";
classList[1]="b2";
classList[2]="b3";
classList[3]="b4";
classList[4]="b5";
classList[5]="b6";

//stores a random number from 0 to 2
randomNum=Math.floor(Math.random()*6)

//grabs the div box from the id
testBox=document.getElementById('banner')

//assigns the div box a random class name from the array 
testBox.className=classList[randomNum]
}