What are microorganisms? List their different types.

Explanation

**Step-by-Step Solution:**

1. **Definition of Microorganisms:**
- Microorganisms are very small living organisms that cannot be seen with the naked eye. They are so tiny that we need a microscope to observe them.

2. **Characteristics of Microorganisms:**
- Most microorganisms are unicellular, meaning they consist of a single cell. This characteristic distinguishes them from multicellular organisms.

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