I'm pretty sure the idea came from... dang, can't think of the site now. I saw it here as "Juice Box Geometry" as well. Students had juice, cereal, and tissue boxes and measured with partners to find surface area and calculate volume. Next they used their measurements to design a net for the box, labeling the parts and dimensions. Then the boxes were carefully opened and pulled apart (juice boxes over the sink, please!), remeasured, and matched up to the nets. Volume was recalculated and compared to original figures -- how/why different? Why was the package designed the way it was? Could you improve on the design or increase the volume?
If I kept my Math students (they come from all three 5th grade classrooms) I thought it would be cool to transfer their nets and measurements into an Art lesson -- design your own product and packaging!