Define the following.
Stimulus

Explanation

**Step-by-Step Text Solution:**

1. **Definition of Stimulus**: A stimulus is anything that produces a response in an organism, or in a cell or tissue of an organism.

2. **Understanding Response**: The key aspect of a stimulus is that it triggers a response. This means that when an organism detects a stimulus, it reacts in some way.

3. **Types of Stimulus**: There are two main types of stimuli:
- **Internal Stimulus**: These are stimuli that originate from within the organism. Examples include feelings of pain or hunger. For instance, if you feel pain, your body recognizes this internal stimulus and you may respond by expressing discomfort or seeking relief. Similarly, if you feel hungry, you will respond by seeking food.
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