cant find my first child when walking down the DOM tree

cant find my first child when walking down the DOM tree

For some reason i cant find my first child.

$("p.menu_head_expand").click(function()
{

This works:
$("#siblingI a:first-child").css({color:"red"});


But this doesn´t:
$(this).siblings("div.menu_body a:first-child").css({color:"red"});


});

The tree where i want to find the first
<a>
:"ball"

<p class="menu_head_expand"><a class="animate expand" href="#">behandling</a></p>
<div class="menu_body" id="siblingI">
<a class="animate" href="#">ball</a>
<a class="animate siblingII" href="#">ring</a>
<a class="animate siblingII" href="#">token</a>
</div>