You made lasagna last night, and it was delicious. You saved the leftovers by wrapping aluminum foil over the top of the pan, and putting it in the fridge.
When you get hungry today, you go to look for that lasagna, but it looks like the lasagna was getting hungry too! It's eaten tiny holes in the aluminum foil that was covering it.
Why does this happen?
There are really two things responsible for those holes in the aluminum foil: the acidic nature of the lasagna and some curious properties of aluminum.
Lasagna gets most of it's acid from the tomatoes in its sauce. Almost everything you eat is at least slightly acidic, but tomatoes are especially so. This acid wouldn't be a problem for a glass container, or stainless steel, or plastic wrap, or for most of the materials we use to store and prepare food. Aluminum, though, is especially vulnerable to acid.
Oxide layer
Here's why. Most metals form a protective layer on their surface called an "oxide layer." Aluminum forms an oxide layer too, but it is very thin, allowing the tomato's acid to easily break through. This lets the tomato sauce dissolve the aluminum, and it creates grey or black chemicals on top of the lasagna that taste very bad.
The same thing can happen if you use aluminum cookware for acidic sauces. Sauce made in an aluminum pot will be grayer and not as tasty as sauce made in other cookware.
