We humans are responsive organisms. Aroma of a freshly cooked dish makes our mouth water, loud thunder makes us jump in our seat and stepping on a nail causes intense pain. We sense the changes in our environment (both internal and external) with the help of special sensory receptors. These environmental changes, called stimuli, once detected by the special sensory cells, are conveyed to the brain in the form of nerve impulses. The meaning of each stimulus is interpreted in the brain and appropriate order is sent to the body parts for its appropriate response to ensure well being.