var color = 0;
var suma = 5;

function aparecer()
{
	var obj = document.getElementById('escondido');
	obj.style.display = "block";
	color += suma;
	if (!(color>=105)) {
		obj.style.filter = 'alpha(opacity='+color+')';
		obj.style.opacity = color /100;
		obj.style.MozOpacity = color /100;
		obj.style.KHTMLOpacity = color /100;
		window.setTimeout ("aparecer();", 25);
	}
}

function desvanecer() 
{
	var obj2 = document.getElementById('escondido');
		color -= suma;
	if (!(color <=-5))
	{
		obj2.style.filter = 'alpha(opacity='+color+')';
		obj2.style.opacity = color /100;
		obj2.style.MozOpacity = color /100;
		obj2.style.KHTMLOpacity = color /100;
		window.setTimeout ("desvanecer();", 25);
	}
}
	
function llama()
{
	var color = 0;
	if (color<=0)
	{
		aparecer();
	} else {
		desvanecer();
	}
}