First thing: you seem to be acting like a child. I'm going to ignore that and address your actual question, but be aware that other people might react badly to the way you wrote your comment.
« ses » and « les » are different words and are not interchangeable. « ses » means "his" or "her": « ses ongles » - "his/her nails". « les » means "the" (plural): « les ongles » - "the nails". You have to use « ses » to change the sentence from a general one ("the nails are red") to a specific one ("his/her nails are red").
Yeah, I was angry. Anyway, my point is in French la/le/les are used to refer body parts. Eg.: Le chat est sur les genoux. It is the way I have seen most of the time in duo. It may be ambiguous but I have seen on duo, that this the way the French are most likely to refer to their body parts.
The sentence would be « Elle a un chat sur les genoux » - "She has a cat on the knees", where it can only be her knees; she can't have a cat on someone else's knees. If you are given a sentence « Les ongles sont rouges » - "The nails are red" how can anyone tell whose nails are being talked about? That's why it's « Ses ongles sont rouges ». We can safely assume that she was just being talked about before so we know whose nails they are. So basically, in French the person is mentioned once at the start of the sentence and they don't feel the need to refer to them again, whereas in English we prefer to make it explicit by mentioning the person twice. She has a cat on her knees. She has a pain in her shoulder, etc.