In my parent's house, we used to store some plastic salad boxes in a wooden cupboard in the cellar. Before storage theses boxes have been emptied, washed and stacked into one another. After some time, at least half a year of longer, we found some of these boxes glued together, where their walls touched each other. To me it looks like there was a chemical reaction going on. Can somebody please explain to me what exactly is going on there?

If I make a box out of mirrors, then open the box in order to to let light in, then I close the box. Why does light not stay stored in the box?