Elizabeth I would like you try an experiment. Gather together some different nails. If you can, find a stainless steel nail, steel nail, iron nail and as many different nails that you can. Try and get two of each. Find two jam jars and put one set of clean nails in one jar and the other set of clean nails into the other jar. Why do I suggest cleaning the nails?
Fill the first jar with tap water and the second with water that has been boiled and cooled. Cover both with cling film and leave for about 7 days. What happens ?????
Relook at what you did at the start. What did you see in the tap water that you added? How did this compare to the boiled water? What did you see when you boiled the water?
Most of the stains on metals are caused by interactions with water and oxygen.
When steel and iron are attacked on their surface by the oxygen from the water you get things called oxides created as the oxygen (a very reactive gas) reacts with the metals surface. For most metals the compound (oxide) that is formed is fairly ‘soft’ and is washed away creating new sites for oxygen attack. With stainless steel it is the chromium in the stainless steel that reacts with the oxygen creating an invisible layer of Chromium Oxide and this is such a hard substance that no other substance can stain the steel.
If you rub the stainless steel implement hard with a scraper you might get rid of the strong oxide and create a stain by attacking the surface of the steel with another substance. Try it (with permission).
Elizabeth and others, quite a detailed answer. If you want to ask further questions please ask.