If you buy a pot pie at the store, the cooking instructions say to fold a strip of tinfoil around the edge before putting it in the oven. I've seen what happens if I forget to do this: the edge gets all burned. Apparently the foil helps keep some of the heat off.
Something strange happened to a friend of mine a while ago, though: she had never made one of these before, and she misread the instructions and put a sheet of tinfoil over the entire top of the pie. It turned into a bit of a mess; after baking, the whole thing was only about half cooked!
That makes me wonder: what exactly is the foil doing? It's nice and shiny, and if this were being cooked under a really powerful heat lamp I could understand how it would reflect a lot of the light away, but this is a conventional oven. Heat is transferred from the air into the food by convection, and aluminum is a very good conductor of heat, so intuitively I'd think it would do practically nothing at all to keep the heat off. But clearly it's actually being very effective, so what's going on?