The difference is because some of us have silver fillings in our teeth. It turns out that aluminum atoms lose their electrons very easily. In the presence of our mildly acidic saliva, which acts as a catalyst, we have what amounts to a crude electric battery. Electrons flow from the aluminum to the silver amalgam filling. Aluminum foil will begin to decompose in the presence of many other acidic substances in a process called oxidation.
Acids like to oxidize obliging metals.
Tell any cooks you know to never wrap a meatloaf glazed with ketchup or tomato sauce in aluminum foil for storage. After several hours the result of this contact is a grayish-black disgusting mush of aluminum oxide. Chemistry solves that problem, too. Giant Bubbles Kids, you can make gigantic soap bubbles with a mixture of liquid soap or detergent, glycerin, and water. The glycerin is the secret ingredient that adds strength to the bubble solution.