Water is made up of the elements hydrogen and oxygen. The smallest particles of matter are called atoms. Remember the carrot from the other chapter. If you continue to chop a carrot into smaller and smaller pieces, eventually you would reach a point where you could not cut up the carrot anymore, but still have carrot.
occult blood test. a test for the presence of microscopic amounts of blood in the feces secondary to bleeding in the digestive tract. The minimal amounts of occult blood normally passed into the GI tract are not usually significant enough to cause a positive result for occult blood testing.
Occult means you do not know there is blood in the stool, i.e., you cannot see bright red recognizable blood like you might see if you had bleeding from hemorrhoids. Testing for fecal occult blood is commonly used as a cancer screen for colon cancer. A positive test can mean other conditions however. A review article, Rockey DC: Occult gastrointestinal bleeding. N Engl J Med 1999;341:38-46 addresses some questions you might have if you turned out to have a positive test.