Mr McGregor

Stephen at some point picked up that in the story of Peter Rabbit, Mr McGregor chases Peter around. So now any time that he hears the words Peter Rabbit or Mr McGregor, he raises his hands up "I'm-gonna-get-you" style, says "Rooarr!" and its time for little Mr McGregor to chase Papa around the house for a few minutes. Peter Rabbit actually gets mentioned quite often, so this happens sometimes several times in a single day.