Industrial paper packaging boxes
Cardboard and other paper-based materials (paperboard, corrugated fiberboard, etc.) can have a post-primary life as a cheap material for the construction of a range of projects, among them being science experiments, children's toys, costumes, or insulative lining. Some children enjoy playing inside boxes.
A common cliché is that, if presented with a large and expensive new toy, a child will quickly become bored with the toy and play with the box instead. Although this is usually said somewhat jokingly, children certainly enjoy playing with boxes, using their imagination to portray the box as an infinite variety of objects.