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A stain is a discoloration that can be clearly distinguished from the surface, material, or medium it is found upon. They are caused by the chemical or physical interaction of two dissimilar materials. Staining is used for biochemical research, metal staining, and art (e.g., wood staining, stained glass).
Types of stains 
There can be intentional stains (such as wood stains or paint), indicative stains (such as food coloring or adding a substance to make bacteria visible under a microscope), natural stains (such as rust on iron or a patina on bronze), and accidental stains (like spilling ketchup on your shirt).
Different types of material can be stained by different substances, and stain resistance is an important characteristic in modern textile engineering.
The primary method of stain formation is surface stains, where the staining substance is spilled out onto the surface or material and is trapped in the fibers, pores, indentations, or other capillary structures on the surface. The material that is trapped coats the underlying material, and the stain reflects back light according to its own color. Applying paint, spilled food, and wood stains are of this nature.
A secondary method of stain involves a chemical or molecular reaction between the material and the staining material. Many types of natural stains fall into this category.
In many cases, stains are affected by heat and may become reactive enough to bond with the underlying material. Extreme heat, such as from ironing or dry cleaning, can cause a chemical reaction on an otherwise removable stain, turning it into a chemical compound that is impossible to remove.
Various laundry techniques exist to attempt to remove or minimize existing stains, and stain removers are an important type of chemical in laundry detergents. Some stand-alone stain removers also exist.
Food and drink stains are the most common types of stains that people search for on the internet. This is due to the variety of food and drinks and the variety of materials that the stains occur on.
See also 
- Understanding Wood Finishing: How to Select and Apply the Right Finish.Bob Flexner. Reader's Digest, 1999. Pg 121. ISBN 0-7621-0191-1, ISBN 978-0-7621-0191-7
- Staining Procedures Used by the Biological Stain Commission: Published for the Biological Stain Commission. George Clark, James W. Bartholomew. Williams & Wilkins, 1981. Pg 32-33, 45. ISBN 0-683-01707-1, ISBN 978-0-683-01707-6
- Architectural Metals: A Guide to Selection, Specification, and Performance. L. William Zahner. Wiley, 1997. Pg 101. ISBN 0-471-04506-3, ISBN 978-0-471-04506-9
- Materials Performance. NACE International. Published by National Association of Corrosion Engineers, 1987. v.26 1987 Jul-Dec, pg 33
- The Stain Removal Handbook. Max Alth, Simon Alth. Hawthorn Books, 1977. Pg 22 ISBN 0-8015-7071-9, ISBN 978-0-8015-7071-1
- Matt Oden, Food and Drink Stain Removal http://remove-stain.com/stain-type/food-and-drink-stains
Further reading 
- Stain & Spot Removal Handbook: Consumer guide. by the editors of Consumer Guide.. Skokie, Ill:Beekman House, 1981. 9780517316832
- Zia, Stephanie. Stain Removal. London: Hamlyn, 2005.Distributed in the U.S. and Canada by Sterling Pub. Co., 2005. 9780600611240
- Soto, Anne MarieStain Rescue!: The A-Z Guide to Removing Smudges, Spots & Other Spills By good Housekeeping Institute (New York, N.Y.). Published by Sterling Publishing Company, 2007 ISBN
- Mendelson, Cheryl Laundry: The Home Comforts Book of Caring for Clothes and Linens Simon and Schuster, 2005 ISBN 978-0-7432-7145-5
- Remove Stains Web Database for Stain Removal