Ultimately, the scientific reason you should never bake cookies on a foil-lined baking sheet is simple: The parts of your cookie dough that come in direct contact with the foil are exposed to more concentrated heat than the rest of your cookies, thanks to aluminum's natural conductor properties.

Should I line my baking sheet with aluminum foil?

Aluminum foil is safe to put in the oven, making it great for lining baking sheets. But it's not recommended to use foil to line the bottom of the oven to catch spills and drips. The best foil uses include: Cooking food on the grill.

What does lining a baking sheet with foil do?

Lining a Baking Pan with Foil

This method of keeping things from sticking to the pan is used most often for brownies, bar cookies, and fudge. The additional advantage of this method is that it allows you to easily lift the bars out of the pan before cutting them. Tear a piece of foil that is larger than the pan.