External stimuli are any changes or signals in the environment that can influence an organism's behavior or responses. These stimuli can come from various sources, such as light, sound, temperature, or chemical signals. For example, bright light can cause a person to squint, while loud noises may prompt someone to cover their ears. Organisms, including humans and animals, constantly interact with external stimuli to adapt and survive. The ability to respond to these stimuli is crucial for activities like finding food, avoiding danger, and communicating with others. For instance, a dog may bark in response to the sound of a doorbell, indicating its awareness of an external change.