Possible Design Lab?

I am a big fan of New Scientist magazine - it is a very good read. At the end of the magazine there is a 'Last Word' column where readers can pose scientific questions so that other readers can offer answers or suggestions. The Last Word is so popular that they even produce books full of readers questions and answers. One such question caught my eye. It asked, why do cardboard fruit juice or milk containers bulge when they are empty ...