How come girls have periods?

Really? You don't know?

It's because of their genetic make up, having to carry a baby and all means they needs to continue having periods so the the uterus is ready for the next egg and so on, otherwise there would just be a continual build up of blood and would break the inner lining of the womb.

Y do guys voice get deeper? ITS CALLED PUBERTY

Bcuz they're the ones who have babies. Gross conversation :P

It's all about hormones and getting a young lady ready for childbirth later in her life!