Plastic food container photographed using the technique described here.
Basically, hold a hard plastic object in front of an LCD screen (which is polarized) and take a picture with a polarized camera lens. The stress points of the plastic are revealed.
"What you're seeing in the plastics is actually the alignment of the molecules and the orientation imposed on them by the molding process (in most of those items, injection-molding). As the hot plastic is injected, the long molecules will align with the direction of the flow. Using this technique, you can often "see" the anisotropic properties and find the weak points of a plastic part." -ewilhelm