//鼠标离开菜单，二级菜单动画收缩起来。        
        aLi[i].onmouseout=function(){
            var oSubNav=this.getElementsByTagName('ul')[0] ;
            if(oSubNav){
                var This=oSubNav ;
                clearInterval(This.time) ;
                This.time=setInterval(function(){
                    This.style.height=This.offsetHeight-16+"px" ;
                    if(This.offsetHeight<=0)
                    //This.offsetHeight=0 ;
                    clearInterval(This.time) ;
                },10)
            }
            
                       
        }
        
       
为什么我在这里加上了This.offsetHeight=0;(上面注释的语句)就不能收缩回去了呢？