Saying 'Sarah finds that joke funny' is different to the act of Sarah laughing at the joke - the first can be analysed to find out if it is true or false, the second isn't something we can analyse in the same way. In the same way, a cognitivist argues that a moral judgement like 'murder is wrong' can be made, whereas a non-cognitivist would claim that it's the same as saying 'Boo! Murder!'