In biology, a stimulus is a change in an organism's surroundings that causes the organism to change its behavior in order to make the environment more satisfactory. For instance, hunger motivates animals to seek food, predators stimulate prey to run away or hide, and falling temperatures encourage creatures to seek shelter or find warmth in other ways.
When something in the environment stimulates an animal, its central nervous system, the brain and spinal cord, organizes the response. The stimulus is sensed by the animal, and its sensory neurons carry that information to the central nervous system. This directs the motor neurons to respond in a specific way, and the animal carries out that order.