Any change in the environment that causes a response in an organism is called a stimulus.

Here's why:

* Stimulus: A stimulus is a detectable change in the internal or external environment of an organism. It can be anything from a change in light, temperature, sound, or even the presence of a predator.

* Response: The organism's reaction to the stimulus is called a response. Responses can be physical (like moving away from a source of heat), chemical (like releasing hormones), or behavioral (like changing eating habits).

Let me know if you'd like to learn more about stimuli and responses!