[flexcoders] Creating tooltip for datagrid column header(s)?

[candysmate]

I wish to create tooltips for datagrid column headers.

My datagridcolumn for the required tooltip is:

<mx:DataGridColumn
headerText="M"
width="26"
textAlign="center"
editable = "false"
headerRenderer="myComponents.moveInactiveTooltip">
 <mx:itemRenderer>
   <mx:Component>
        <mx:HBox horizontalAlign="center" click="outerDocument.moveToActive();">
          <mx:Image height="16" width="16"
source="@Embed('/images/backwardicon.png')" toolTip="Click to make
'Active'" >
          </mx:Image>
        </mx:HBox>              
   </mx:Component>
  </mx:itemRenderer>
</mx:DataGridColumn>

my mxml component is:

<?xml version="1.0" encoding="utf-8"?>

<mx:ToolTip xmlns:mx="http://www.adobe.com/2006/mxml"; text="Move to
inactive">
        
</mx:ToolTip>

but Flex (2.01) dies not like this and reports:
'Could not resolve <mx:ToolTip> to component implementation'

What have I done wrong?