Actually I can't, it just isn't in me.
(This is a double play on an old ad on tv here for, as I recall it, some kind of corned beef: the voice over said to the tinned marvel: "Say oink". The answer: "Moo". Again: "Say oink". Answer: "Moo". Questioner: "Why won't you say oink?". The palaver-ending replique: "It just isn't in me. ).