Living things respond to a stimulus, which is a change in the surroundings. Some common stimuli are noises, smells, and things the people see or feel, such as a change in temperature. Animals often respond to a stimulus by moving. Because plants can't move around in the same way animals do, plants have to respond in a different way. Plants can respond to a stimulus by growing either toward or away from the stimulus. The growth of a plant in response to a stimulus is called a tropism. This article discusses tropisms and describes an activity on the effect of gravity on plants.