We know it by so many names. Aunt Flo. Crimson tide. Got the painters in. The Blob. Bloody Mary. My friend. The curse. Code Red. My period.
However you describe it, chances are your preferred nickname for menstruation is designed to camouflage what's really going on - which is amazing, considering it happens to virtually every woman of childbearing age around the world. That's 26% of the population.
It's time to break the silence. Here in the UK, menstruation may be an annoyance. It may even be painful. But for many girls and women around the world, it carries much more serious consequences.
Among the most extreme is the custom of chhaupadi, an extreme form of exclusion practised in rural parts of India and Nepal, where menstruating girls and women are sent out to cowsheds and fed boiled rice thrown to them as we would feed an animal.
Nothing has changed
Exclusion takes many forms - from girls who drop out of school because there is no private toilet or running water so they can care for themselves, to young women who believe they cannot bathe, or cut their hair, or cook, or touch plants for fear of wreaking harm on themselves or those they love.
Gloria Steinem wrote satirically in the 1970s of what would happen if men had periods - they'd be celebrated, researched, supported. Thirty-five years later, we're still making the same jokes, and not much has changed.
If we can't talk about our periods, how can we expect girls in more traditional societies to voice their rights and needs?
This week, 28 May, is Menstrual Hygiene Day, declared by a group of NGOs that work with girls and women around the world and see first-hand the impact of such myths and a lack of basic services.
Programmes like working with schools to develop separate toilet blocks for girls, or teaching young women to sew reusable cotton pads if their families can't afford disposable, can prevent infection and mean the difference between a girl staying in school and dropping out.
Kumie Abdissa (right), 19, a student in Oromia, Ethiopia, with a friend outside her school. She has seen friends drop out rather than try to manage periods at school.WaterAid/ Behailu Shiferaw
"Every one of us here has dreams, but look how close girls are to losing out on their dreams. We are not competing with boys on level grounds," said Kumie Abdissa, 19, a student at Dilela High School in Oromia, Ethiopia, told WaterAid researchers. She told the story of a childhood friend who got her period in school, and was so humiliated by her stained clothing and teasing from classmates that she left and never returned.
"All she needed was a toilet with water and maybe a sanitary pad and she could have been here with us right now," Kumie said. "Having access to a clean toilet and water in school means life to girls like me. Having access to a clean toilet and water in school could have kept girls like my friend in school and their dreams on track."
Change is happening
Kumie's experience in Ethiopia is not unique. A UN agency estimates 1 in 10 girls in Africa miss school during their periods.
From our research, we know that 66% of girls-only schools in India do not have functioning toilets. In Niger, 77% of girls have nowhere at school to change their sanitary pads. In South Asia, 32.5% of schoolgirls had not heard about menstruation before their first periods, and nearly all of them - 97.5% did not know the blood was coming from their uterus.
WaterAid works in more than a dozen countries, often with other agencies, to teach schoolgirls and their families about how to take care of themselves properly during their periods. School projects range from building private, gender-separate toilets and taps for washing to creating hygiene clubs where girls learn how to sew themselves reusable pads. Change is happening, but it takes time, effort and political will, here at home as well as abroad.
Later this summer, WaterAid will present this petition as a reminder to world leaders that safe water, a private toilet and soap and water for washing hands matter even more to girls on their periods, and everyone, everywhere deserves them. Join us, and on 28 May, celebrate #menstruationmatters.