Any regular watcher of PMQs will be well-acquainted with David Cameron's temper. He lost it today with the Shadow Chancellor Ed Balls, who seems to have winding up the Prime Minister down to a fine art.
Cameron was rebuked by the Speaker for using unparliamentary language after he referred to the "muttering idiot sitting opposite me". Forced to withdraw the comment, Cameron said that he would replace the word "idiot" with "the man who left us this enormous deficit".
It's water off a duck's back - in the past, Cameron has called Balls "the most annoying person in modern politics".