Do you know why the aluminum foil has 2 different sides?

No significant difference: While the shiny side reflects more heat, and the dull side absorbs it, the difference is so minimal that it doesn’t significantly affect cooking.
Practical Considerations: The main difference comes from the manufacturing process. The shiny side is the result of the final pass through rollers at the factory, while the dull side is a result of the foil being on the inside during the manufacturing process.
What Should You Do?
It’s not critical: For most cooking tasks like wrapping food or lining pans, you can use either side without any noticeable impact on the outcome.
When using for roasting or baking: If you’re trying to reflect heat away from food (e.g., if you’re baking something delicate), you can use the shiny side out. Conversely, if you want to retain more heat and crisp food (e.g., when roasting vegetables), the dull side could be placed facing the food.
Conclusion:
While there are slight differences, in most cases, it’s okay to use either side for cooking. Choose the shiny or dull side based on your preferences, but rest assured that it won’t dramatically affect your meal!