How do I make my girlfriend to be barefoot more often?

I love my girlfriend, she's great. She gots some really good things about her and one of them is her feet, her feet are pretty nice and she always keeps them clean. I want her to go barefoot more often, she's always wearing socks and closed shoes, even in the summer. I wouldn't mind so much if she doesn't like to be barefoot but just to try.
How should I say it?
How do I make my girlfriend to be barefoot more often?
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