为什么鼠标滑过颜色这么写不对啊，划过的时候整个表格都变色了

//鼠标滑过的背景颜色
     function onload2(){
        var table=document.getElementById("table");
        var trs=table.childNodes;
          for(var i=0;i<trs.length;i++)
          {
              trs[i].onmouseover=function()
              {
                  //alert(trs.length);
                  this.style.backgroundColor="#f2f2f2";
              }
              trs[i].onmouseout=function()
              {
                  this.style.backgroundColor="";
              }
          }
    }