One way in which organisms maintain homeostasis is by detecting foreign cells and particles like pathogens and cancer cells. once the pathogen is detected and identified, other systems in the organism's body can attack the invader, thus keeping the organism healthier. cells of the human immune system are finely tuned to recognize and respond quickly to disease-causing organisms.

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms

the rocks are classified by the geological process they are formed by. the three main groups of rocks are igneous, metamorphic, and sedimentary. minerals are not they are classified under the chemical classes they are in. these groups include silicates, sulfates and carbonates.