A full discussion of the metaethical issues raised by this argument  would take us too far afield  (see Parfit 2006). But one strategy of  reply is to argue that the realist has already answered the question.   What it is for such normative facts to be normative just is  that they require that you act in some way.  If you’re asking of  normative facts in general why you should act in accordance with them,  then you don’t fully understand what normative facts are.