"Put a piece of aluminum foil with a copper coin on it into a glass of water, and let it stand for a day. After this the water looks cloudy and at the place where the coin was lying the aluminum foil is perforated.
This process of decomposition is known as corrosion. It often occurs at the point where two different metals are directly joined together. With metal mixtures (alloys) it is particularly common if the metals are not evenly distributed. In our experiment the water becomes cloudy due to dissolved aluminum. A fairly small electric current is also produced in this process."