I don't know what it is, but there's something about a girls' feet when she's been wearing some old flats all day. The smell her feet make inside her flats is incredible. No matter which pair of flats my girlfriend wears, OMG, they smell amazing inside when she slips them off, and so do her feet. It's an arousing scent, the stronger the better. A former girlfriend wore flats all the time and she was the same way.