" My dear Caroline,” I said. “There’s no doubt at all about what the man’s profession has been. He’s a retired hairdresser. Look at that moustache of his.” Caroline dissented. She said that if the man was a hairdresser, he would have wavy hair—not straight. All hairdressers did. "

― Agatha Christie , The Murder of Roger Ackroyd (Hercule Poirot, #3)