How to retrieve parent node using XQuery?

I am using XML and XQuerie. I usually use an XPath expression relative to a parent node to retrieve its child node. But, I am not sure how to do the opposite meaning if I have a child node, how do I retrieve its parent node.

<node id="50>
  <childnode1 childid="51" />
  <childnode2 childid="52" />
</node>

If I have the node <childnode1 childid="51" />, how do I retrieve its parent: <node id="50>