A change in an organism's environment that affects its activities is called a stimulus.

Here's why:

* Stimulus: A stimulus is anything that causes a reaction or response in an organism. It can be a change in temperature, light, sound, chemicals, or even the presence of another organism.

* Response: The organism's reaction to the stimulus. This can be a behavioral change, a physiological change, or both.

Example:

* Stimulus: A decrease in temperature

* Response: A mammal may shiver to generate heat and maintain its body temperature.

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