Daughter-in-law has been away for two years and is a hydraulic technician. What happened when her nipples and buttocks turned from pink to black after she came back?

Scientifically speaking, it is due to melanin precipitation.

1. I didn't do a good job in sun protection. I often ate soy sauce and was affected by melanin.

2. Irregular living habits, often eating greasy food and staying up late, endocrine disorders, and estrogen confusion lead to blackening.

3. Friction causes underwear not to fit, underwear is too tight, riding a bike often, being too clean and often cleaning, and finally it is caused by sex life.

4. Natural discoloration when you get older