1. A stimulus is any change in an organism's environment that causes it to react, while a response is how the organism reacts to the stimulus through a change in behavior.
2. There are two types of stimuli: external stimuli that come from outside an organism like feeling cold, and internal stimuli that come from inside an organism like feeling hungry.
3. Animals can respond to stimuli through learned behaviors that are taught, like a mother bear teaching her cubs to fish, or through instincts that are inborn reactions like an animal's natural instinct to run from danger.