Explain that I only have a charisma score of 8, because I thought it would make more sense (belonging to the "nerd" class, and all), to dump most of my points into intelligence. I'd then tell him that, unless he had a dice ready, I'd have no ability to roll and see if I could convince him, and would have to assume I wouldn't roll the 20 necessary to convince him.
I'd then mutter under my breath about how it's a good thing my intelligence stat raised my deception skill up to 20, making it easy to convince him I was much more nerdy than I actually was.