A frequent problem in cooking is what to do with a partially consumed avocado.  Cut avocados are quickly oxidized by contact with the air into an unappealing brown color.  I recently saw an article suggesting storing cut avocados in water, and it does work.  Because avocados are high in fat, they do not mix much with water.  The water keeps out the air but does not get absorbed much by the avocado.  The photo shows an avocado which was left exposed to the air for some hours (thus there is some brown) but then has been in water in the refrigerator for about 18 hours. Eventually, the cut surface in contact with the water does get a bit soggy, so this is best for use of the remaining avocado in a day or so.