Any time he is asked the question, Victor Matfield cites Paul O'Connell as his toughest opponent.
Speaking on BT Sport's Rugby Tonight, Matfield told of how one particular game in Dublin played a major part in the formation of that opinion.
Matfield does not name the game specifiically, but we're pretty sure it's the November 2009 game at Croke Park. That was, as the former Springbok says, an especially misty day.
Ireland were hugely successful in disrupting the South African lineout during that encounter. How exactly they achieved this, Donncha O'Callaghan was able to explain.
Paul O'Connell spent the week listening to the Count in Sesame Street - you know [imitates Sesame Street character], 'One, aahh, aahh, aahh. Two aahh, aahhh, aahh.' - in Afrikaans so that he could get an ear on your lineout calls a bit.
That's the kind of detail some guys will go to.
You can watch Matfield and O'Callaghan on BT Sport below.
— Rugby Tonight (@RugbyTonight) December 28, 2015