In 1999 the world population passed the 6 billion mark. In this lesson, students predict when it will reach 7 billion. To do so, they use an on-line counter (from the PBS site, "Six Billion and Beyond") that simulates the changing world population.

They time the counter to find how long it takes for the population to increase by, say, 50 or 100 people. They use that measurement to predict how long it would take for the population to increase by 1 billion. Students discuss the reliability of their predictions, compare them to past trends, and discuss social factors that can affect population growth.