How can I let my girlfriend know that her thong sticks out all the time?

My girlfriend wears thongs almost everyday and it always sticks out in public and I see people staring sometimes its awkward for me. Its not that I don't like her wearing thongs, I do like seeing it but its too often in public and I see people stare at her. How can I tell her this, I don't want her to get mad at me or make it awkward? She isn't trashy or anything, I don't think she does it on purpose because she always pulls her pants up but it always ends up showing.
How can I let my girlfriend know that her thong sticks out all the time?
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