Just because I happen to like a woman's tush. This would have been more effective if you could see the "T" above her jeans though. This pic is overexposed when it didn't need to be.
Us guys can spot this bad habit from a mile away... IMO The crop and the location take away from this picture. If you couldn't capture this in a beach/pool environment, I might have thrown a blanket or beach towel out on the back lawn in a nice sunny area. Have the subject laying face down maybe reading a book held by the right hand and the left hand picking the floss as you have here. Take the shot from a tripod near the subjects feet and kind of catch the whole body and still keep the animinity by not catching the subjects face. I would try to catch the dof where the hand/string are in focus but the rest of the pic a little fuzzy. If you don't have a lawn availbable this could have been done in a bedroom as if the subject was dressing for the beach/pool kind of thing. Just my thoughts, I think it would be better with more objects in the picture.
The colors here are nice. The background is a little bland and doesn't add much to the composition. The contrast could be bumped up a little bit - it all seems slighly washed out. I'm not sure if it's caused by her position, but it seems like the rotation is slightly off or maybe it's just that the strap on the right side is higher and the butt cheek is lower.