divs = ['t1','t2'];
function hideDivs() {
for (var i=0; i<divs.length; i++)
document.getElementById(divs[i]).style.display = 'none';
}
function showDiv() {
hideDivs(); //hide them all before we show the next one.
var randomDiv = divs[Math.floor(Math.random()*divs.length)];
var div = document.getElementById(randomDiv).style.display =
'block';
//setTimeout(showDiv,12000);
}