Simulate Click-Event in Combobox

Hi,

I'm having a DataGrid with a Combobox bound to it. When I enter a
DataGridCell the comboBox is shown. BUT, to typ something in it or scroll
through the ComoBox-Items I have to click a second time in it.

Now my queqtion is: how can I kind of 'simulate' the Click in that ComboBox,
so that, on the moment the users enters the cell (by MouseClick or Tab-Key),
the second clikc is done automatically?

Anybody got any idea?

Thanks!

Pieter