We have a toaster with a plastic lid. One is supposed to put the lid on only after the toaster has cooled off, to protect the inside of the toaster from dust. But apparently some time ago someone neglected the pictogram and put the lid on while the toaster was still rather hot. Now in the most heat-exposed places, the plastic has become of a clearer transparent color, more irregular shape of surface, and there are a lot of fine bubbles inside, about half a millimeter in diameter. My question is: What is in the bubbles and how did it get there? Did the plastic contain fine dispersed air initially? Or maybe water or some organic components that "boil" at a moderate temperature?