What is a tissue?

Explanation

**Step-by-Step Solution:**

1. **Definition of Tissue**: A tissue is defined as a group of cells that work together to perform a specific function.

2. **Characteristics of Cells in a Tissue**: The cells that make up a tissue are typically identical in both structure and function. This means that they have similar shapes and carry out similar roles within the organism.

3. **Types of Organisms**: Organisms can be classified as unicellular or multicellular.
- **Unicellular Organisms**: These organisms consist of a single cell and do not have tissues because there is no grouping of cells.
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