the innocent smile

Is there such a thing as an innocent smile? A smile without any reason, without any expectation. Does it exist? We smile because (a) something's funny and it would be inappropriate to laugh out loud, (b) we want the person we're smiling at to smile back, or (c) someone is already smiling at us, and we feel the need to smile back. Do we, ever, smile just because? How lucky can someone get to be given this elusive, innocent smile..