We know what the birth of a revolution looks like: A student stands before a tank. A fruit seller sets himself on fire. A line of monks link arms in a human chain. Crowds surge, soldiers fire, gusts of rage pull down the monuments of tyrants, and maybe, sometimes, justice rises from the flames.

But sometimes freedom and opportunity slip in through the back door, when a quieter subversion of the status quo unleashes change that is just as revolutionary. This is the tantalizing idea for activists concerned with poverty, with disease, with the rise of violent extremism: if you want to change the world, invest in girls. (See pictures of women in war.)

In recent years, more development aid than ever before has been directed at women  but that doesn't mean it is reaching the girls who need it. Across much of the developing world, by the time she is 12, a girl is tending house, cooking, cleaning. She eats what's left after the men and boys have eaten; she is less likely to be vaccinated, to see a doctor, to attend school. "If only I can get educated, I will surely be the President," a teenager in rural Malawi tells a researcher, but the odds are against her: Why educate a daughter who will end up working for her in-laws rather than a son who will support you? In sub-Saharan Africa, fewer than 1 in 5 girls make it to secondary school. Nearly half are married by the time they are 18; 1 in 7 across the developing world marries before she is 15. Then she gets pregnant. The leading cause of death for girls 15 to 19 worldwide is not accident or violence or disease; it is complications from pregnancy. Girls under 15 are up to five times as likely to die while having children than are women in their 20s, and their babies are more likely to die as well.

There are countless reasons rescuing girls is the right thing to do. It's also the smart thing to do. Consider the virtuous circle: An extra year of primary school boosts girls' eventual wages by 10% to 20%. An extra year of secondary school adds 15% to 25%. Girls who stay in school for seven or more years typically marry four years later and have two fewer children than girls who drop out. Fewer dependents per worker allows for greater economic growth. And the World Food Programme has found that when girls and women earn income, they reinvest 90% of it in their families. They buy books, medicine, bed nets. For men, that figure is more like 30% to 40%. "Investment in girls' education may well be the highest-return investment available in the developing world," Larry Summers wrote when he was chief economist at the World Bank. Of such cycles are real revolutions born. (See TIME's video "Education in Haiti: A Teacher's Passion and Vision for Change.")

The benefits are so obvious, you have to wonder why we haven't paid attention. Less than 2¢ of every development dollar goes to girls  and that is a victory compared with a few years ago, when it was more like half a cent. Roughly 9 of 10 youth programs are aimed at boys. One reason for this is that when it comes to lifting up girls, we don't know as much about how to do it. We have to start by listening to girls, which much of the world is not culturally disposed to do. Development experts say the solutions need to be holistic, providing access to safe spaces, schools and health clinics with programs designed specifically for girls' needs. Success depends on infrastructure, on making fuel and water more available so girls don't have to spend as many as 15 hours a day fetching them. It requires enlisting whole communities  mothers, fathers, teachers, religious leaders  in helping girls realize their potential instead of seeing them as dispensable or, worse, as prey. (See a brief history of women in power.)

A more surprising army is being enlisted as well. A new initiative called Girl Up aims to mobilize 100,000 American girls to raise money and awareness to fight poverty, sexual violence and child marriage. "This generation of 12-to-18-year-olds are all givers," says executive director Elizabeth Gore, the force of nature behind the ingeniously simple Nothing but Nets campaign to fight malaria, about her new United Nations Foundation enterprise. "They gave after Katrina. They gave after the tsunami and Haiti. More than any earlier generation, they feel they know girls around the world."

And so the word goes out, by text, by tweet, on Facebook, that coming soon to a high school gym near you may be a Girl Up pep rally, where kids can learn what it feels like to carry a jerrican of water for a long distance, or how sending $5 to Malawi can stock a health clinic with girl-friendly materials or buy school supplies. Or how $5 to Ethiopia can make the difference in a girl's not being married when she's 10. And one at a time, a rising generation of American girls helps create the next generation of leaders, for the coming quiet revolutions.