Robert Hooke looked at a piece of cork under a microscope. The little boxes he saw in the cork are called cells.
1. **Explanation:** Cells are the basic structural and functional units of all living organisms. When Robert Hooke observed the cork through a microscope, he identified the small compartments within the cork as cells. This discovery marked the beginning of the cell theory, which states that all living things are composed of cells.
2. **Example:** Just like how a building is made up of bricks, cells are the building blocks of living organisms. Each cell has its own specific function and contributes to the overall structure and function of the organism it belongs to.
Therefore, the correct answer is **cells**.
