A tampon is a simple thing: a wad of cotton and string stuffed into a plastic tube. But not having access to this simple thing is keeping young girls out of school for several days every month.
Freedom4Girls, a nonprofit that provides period products to women in Kenya and now in the UK, was contacted by a UK school recently because teachers had noticed that the same few girls were missing school for about a week every month.
Tina Leslie, who is part of Freedom4Girls, told BBC Radio 4's Woman's Hour that she wasn't shocked to get the school's call. They had an idea that poor girls would have this problem at school.
"If you've got no food, you've got no money for sanitary protection," she said to BBC Radio. "If you have a mum with two teenage girls, that's a lot of money each month when you're on zero-hours contracts, benefits or low income."
Some of the teenagers talked to BBC about their experiences. "I wrapped a sock around my underwear just to stop the bleeding, because I didn't want to get shouted at," one of the girls said. "And I wrapped a whole tissue roll around my underwear, just to keep my underwear dry until I got home. I didn't know what else to do."
Periods are a big problem for homeless women and girls, as well as impoverished people of all genders who have periods, and there are more homeless teenagers than you'd probably think. One out of every eight students in New York City experiences homelessness at some point or another. These kids face so many hardships at school already, and for those who menstruate their periods just add more problems.
Organisations like Freedom4Girls help, but they can't solve the issue altogether. Leslie was able to send sample packs to the UK school that contacted her, but told BBC that she knows it's not a sustainable solution. That's why some advocates are fighting for free tampons and pads in all public bathrooms, so girls like these will never need to miss school over something as simple as a tampon or a pad.