A few weeks ago (MQ Maths and Stats Colloquium, 24 April), I drew some pictures of D-modules and told you how we could view them as representations of the Lie algebra sl(2,C). In this talk, I'll take things one level higher and discuss actions of the category of representations of sl(2,C) on categories; i.e. representations of representations of sl(2,C). Together we'll uncover the beautiful fact that categories admitting such an action are classified by the simply laced Dynkin diagrams.