I think you missed the parent's critique. The saying is not 'back of the napkin', it's 'back of the envelope', or 'on a napkin'. No one uses an old wadded up

I think you missed the parent's critique. The saying is not 'back of the napkin', it's 'back of the envelope', or 'on a napkin'. No one uses an old wadded up napkin that had a drink on it - but plenty of ideas have come from, 'get me that napkin over there, I've got an idea' (a clean napkin...).