In this interactive, investigate a set of stacked bar graphs representing daily population flux in 25 major US cities. In the accompanying classroom activity, students predict whether more people are in their town at noon or at midnight, taking into consideration those who commute out for work, those who commute in, those who live and work in town, and those who do not work. Students estimate absolute number and percentage in each group. Then, they use the interactive. To get the most from the lesson, students should have experience calculating percentage with a calculator.