why  only  boys  have  facialhairs? why girls don't have it

Dear Student,

We humans have both estrogen (female hormone) and androgens (the male hormone). As we attain maturity, depending upon the sex, the ratio of these hormones changes. In males, the androgen increases and leads to secondary sexual characters such as mustaches, beard, low piched voice, muscles etc. In females the estrogen increases and leads to the feminine secondary sexual characteristics. When there occurs an imbalance in the ratio leading to the more production of male hormones in female, they start having noticeable hairs on their face and other parts.

Hope this clears your doubt,

Cheers!!