function changeLayer(num) {
		var $nav = document.getElementById("howto-nav");
		var $imgs = $nav.getElementsByTagName("span");
		
		for(var i=1; i<=2; i++) {
			var $n = document.getElementById("howto-layer-"+i);
			$n.style.display = "none";
			$imgs[i-1].className = "";
		}
	
		var $node = document.getElementById("howto-layer-"+num);
		$node.style.display = "block";
		$imgs[num-1].className = "active";
	}
	
	window.onload = function() {
		changeLayer(1);
	}