Aluminum foil has a shiny side and a dull side. The difference comes from how foil is rolled thin.
Foil starts out as an aluminum sheet, say, about 0.3 mm thick. It’s passed again and again between polished steel rolls, and each pass cuts the thickness roughly in half. But as the foil gets thinner, the top and bottom rolls start pressing right against each other beyond its edges, so part of the force is taken up where the rolls touch. The thinner the foil, the harder it is to put enough force on the aluminum to thin it further.
So for the final pass, two sheets of about 0.02 mm go through together. The rolls only have to get the pair down to 0.02 mm in total, and when the sheets are peeled apart, each one is about 0.01 mm thick: one pass, twice the foil. The outer sides, which touched the polished rolls, stay shiny. The inner sides, pressed against each other, get tiny bumps that scatter the light.
Quite simple & clever I thought.