There is a very convincing and pervasive answer to this question that can be found all over the world, especially in those areas with tropical climates, winter climates, or a proximity to lakes, rivers, streams, and oceans: rust.

Rust is the direct result of the corrosion of iron or metal by oxygen and hydrogen, a process whereby electrons from the hard surface are transferred into the water or moist air.  The degree to which this happens depends on the type of metal and how it has been forged, as well as whether or not the iron is of a type that develops a protective electrochemical coating when subjected to these conditions.

A Damaging Problem 

Because rust is so pervasive in many parts of the United States, it's easy to overlook how damaging a phenomenon this corrosive reaction represents. In the mid-1970's, a study found that the annual cost of dealing with rust was $70 billion, or 4.2% of the Gross National Product (GNP). Among the stranger settings for corrosive water-iron reactions is the human body. The increasing use of pins, pacemakers, hip joints and other alloy insertions has brought with it the challenge of keeping those items intact in a body organism that is mostly water.

Stainless Steel 

Stainless steel, which by definition must contain at least 10.5% chromium, does not rust because that chromium reacts with the outside elements to form a protective layer of chromium oxide.  However, in some cases, a stainless steel surface can become contaminated on the surface with other iron particles, which  can then in turn rust and create that familiar, dreaded red residue.

The Swedish Corrosion Institute spends a great deal of time analyzing the fallout effects of iron interaction with water and air.  For example, their 2000-2001 study of the effect of rust on select makes and models of cars found that SAAB, Audi and Volvo were best in this department, while Toyota, Mitsubishi and Chrysler were at the bottom.