window.onload=function(){
        var oDiv=document.getElementById('mask');
        oDiv.onclick=function(){
            var iHeight=css(oDiv,'height');
            var iTarget=-iHeight;
            var timer=null;
            
            timer=setInterval(function(){
                var iTop=css(oDiv,'top');
                var iSpeed=Math.floor((iTarget-iTop)/1);
                var t=iTop+iSpeed;
                if(iTop<=iTarget){
                    clearInterval(timer);
                }else{
                    oDiv.style.top=t+'500px';
                }
            },10);
        }
    }