Oh my god, I don't want to tell everyone about skeletons. Maybe other monsters... [He has used the word 'monster' with at least two other not-quite-humans already.] Magic's easy. If they ask about yours, I'll just tell them, you're too lazy to practice!

[That isn't quite true in multiple ways, but he trusts himself to be able to rant and complain about Sans's inactive lifestyle at the drop of a hat. Really, the honesty would sell the whole thing.]