JQuery Swap 2 div's z-index

[boothole]

i tryed to change div's z-index.![enter image description here][1]Here is the image link : http://img593.imageshack.us/img593/751/testsl.pngThe divs are pencil and the white curtain in the back of the handwriting.\[code\]else if (pen_state == 1) { var pen_zindex = $("#pen").css("z-index"); var curtain_zindex = $("#curtain").css("z-index"); if (curtain_zindex < pen_zindex) { $("#curtain").css('z-index',pen_zindex); $("#pen").css('z-index',curtain_zindex); } }\[/code\]But what ever i do i can not swap them.Waiting for your helps. thanks a lot.