Why does my function run multiple times after UpdatePanel is loaded

So I want to run some javaScript function after my updatepanel is updated, so I have:

function pageLoad() { 

    var prm = Sys.WebForms.PageRequestManager.getInstance();
    prm.add_pageLoaded(panelLoaded);
}


function panelLoaded(sender, args) {
        alert("foobar");
}

With the above code, if I update the panel one time, "foobar" will be alerted one time; If I update the panel the second time, "foobar" will pop up twice; the third time I trigger the panel to update, "foobar" popped up three times... 4th time pop 4 times so on and so forth....

What caused this??

Thanks~~~