Why do women have periods?
View full lesson: http://ed.ted.com/lessons/why-do-women-have-periodsA handful of species on Earth share a seemingly mysterious trait: a menstrual cycle. We’...
![Why do women have periods?](https://i.ytimg.com/vi/cjbgZwgdY7Q/maxresdefault.jpg)
View full lesson: http://ed.ted.com/lessons/why-do-women-have-periodsA handful of species on Earth share a seemingly mysterious trait: a menstrual cycle. We’...