According to Simply Recipes, the dull side is the non-stick side so that's the side to cook your food. So why is there a shiny side and dull side? It's actually one of the best fun facts out there.

Makers of aluminum foil don't intentionally make a shiny and a dull side for any culinary purpose ... stainless steel or iron with aluminum foil. That discoloration is aluminum oxide, a harmless ...