All living things are able to respond to stimuli in the external environment. For example, living things respond to changes in light, heat, sound, and chemical and mechanical contact.
All living things must be able to respond to what's going on around them. Anything that causes an organism to react or respond is called a stimulus (plural: stimuli). Stimuli can be external (outside the organism) or internal (inside the organism). Let's look at some examples of organisms responding to stimuli.
