I believe that the most important attribute to life is the ability to change the surrounding environment. Something like a rock cannot change its surrounding environment much, unless it is moving into an environment like an asteroid into earth’s atmosphere. Life takes molecules from another area and produces something else. Even viruses fit this, because when they infect a cell, the cell changes to produce more of the virus. Crystals take elements and form them into a pure, organized manner. Parasites remove nutrients from their host and use them to grow. Since this method of classifying life includes metabolism, plants and animals are obviously included.