Many folks think marble and granite are much alike. Actually, granite is a very hard stone, and most varieties of marble are composed of a soft substance derived from sea shells. Marble is easily damaged by cleaning products containing alcohol or acid. Most perfumes and many medicines contain alcohol. Citric acid from oranges, lemons and grapefruit can desolve marble, yet mild acids such as grapefruit also can be used to clean a marble surface. If you have a dirty marble surface, try this: Put a layer of table salt on a small table, then slice a grapefruit in half. Dip the grapefruit into the salt and rub it on the surface of the marble briskly -- in the direction of the grain. If the marble can be cleaned, this will do the trick. And that's the On The House tip for today.