In honor of World Population Day today take a moment and recognize the importance of being counted.  There are six million adolescent girls between the ages of 10 and 19 counted in the world today, but not all those girls have the opportunity to achieve their dreams and make a difference.

Everyone loves to get a letter from someone special. And nothing is more special than a new friend from far away who cares about your health and your happiness. I watched this happen as I saw some very special letters brighten the hopes of girls in the Liberian village of Careysburg last week.

According to this week's essay in TIME Magazine by Nancy Gibbs, the hundreds of thousands of young people mobilizing with Girl Up are an "army enlisted in helping girls realize their potential." Whether you are teenage girl (or boy), reading this right now, together, we are a force for change and a generation of investment.

Girls count. Did you know that some girls in the developing world spend up to eight hours a day walking back and forth to get clean water for their families? Did you know that 75 percent of the children in the developing world who aren’t able to go to school are girls? Did you know that not every girl in the developing world has a birth certificate when they’re born?