What was it like when you got your first period?
Cramps? Stained trousers? Sudden panic that someone might KNOW?
Now imagine what that would have been like if your school were in a developing country, without a proper toilet or sink to wash your hands in.
For millions of girls and young women, this is reality. On International Women's Day, when we celebrate our achievements and what makes us women, it is this single part of being female that in some parts of the world still cripples a girl's chance at a good education.
Because if you can't change your sanitary towel or tampon - if you even have access to one -in private at school, with the ability to wash afterward, you're just not going to go at all.
Girls in the developing world drop out in droves once they reach puberty. Part of that is cultural expectation. Once you're considered a woman, many are expected to marry and begin their own households.
But even for those whose families support them continuing in school, the idea of sharing a toilet with the boys - or, worse, having to use a field or forest nearby - quickly defeats the idea.
Right at this moment, some 800 million girls and women around the world, between the ages of 15 and 49, are menstruating. If you're here in the UK, it's a nuisance, maybe even mildly unpleasant.
But in some countries, you might be blocked from using the household toilet, or community water point, for being 'unclean.'
Take the example of 14-year-old Kasech, a young woman in her sixth year of school in Ethiopia.
"I experienced menstruation this year. At first I was extremely terrified and thought I had some kind of serious sickness. Later, my friend confirmed to me it was menstruation. I didn't tell my mother because I was very shy and ashamed. People think first sexual intercourse causes menstruation," she told researchers at WaterAid, an organisation working on improving access to safe water, sanitation and hygiene in 27 countries including Ethiopia.
Too embarrassed to tell their parents, girls like Kasech will resort to using old rags, or stealing grain from home and selling it at market to buy sanitary towels.
Many will stop going to school entirely. Only 45 per cent of schools in the least-developed and lowest-income countries have reasonable toilets for their students. In sub-Saharan Africa, around 56 per cent of boys will finish their schooling, but only 46 per cent of girls do.
A 2010 UNICEF study in Malawi showed that just 37 per cent of girls finished primary school - because of poor or nonexistent toilets, few female teachers and overarching attitudes towards girls' education.
That menstruation is so taboo makes it worse. In the UK, we're embarrassed to talk about it, but girls can learn on their own - online, in books or from adults they trust. In many countries the subject is so shrouded in mystery and taboo that even adults don't know the facts. Girls start bleeding and don't know why, so they are afraid.
Depending on where she is in the world, a menstruating girl or woman may be kept away from cooking, small children or from going to temple. She may even be shut in a cowshed to keep from 'contaminating' the family.
Attitudes and superstitions are hard to change. But we can remove one barrier by focusing on putting taps and toilets into schools and making sure they are safe and private for girls, and teaching girls about menstruation and how to care for themselves. This opens the door to girls staying in school longer.
In Kasech's case, there is good news. With a WaterAid partner organisation, her school began a sanitation and hygiene programme, teaching them how to sew reusable cotton pads and how to look after themselves, and giving them a safe place to do so.
Those small changes will help these girls to stay in school longer, and allow them to complete their education. And who knows where that might take them?
At WaterAid we're trying to get everyone, everywhere access to basic safe water and toilets by 2030, in homes, schools and health centres. It can make a difference to girls like Kasech. And on 8 March, that's worth thinking about.
As an ambassador of Water Aid I can truly say it's a great honour to help females in need and the more we can help the better it will be.
Students in a hygiene class in Geyi village, Ethiopia. Water taps, separate girls' and boys' latrines and lessons in proper hygiene help improve students' health and help young women manage their periods so they are more likely to stay in school.
Credit: WaterAid/Behailu Shiferaw