Scientists classify sponges as simple animals and not plants because of several key differences:

1. Lack of Chlorophyll: Sponges lack chlorophyll, the pigment that plants use to photosynthesize and create their own food. This means they cannot produce their own energy from sunlight like plants do.

2. Heterotrophic Nutrition: Sponges are heterotrophic, meaning they obtain nutrients by consuming other organisms. They filter water for food particles like bacteria and small organisms.

3. Multicellular Structure: While simple, sponges are multicellular organisms, composed of different types of cells. They do not have the complex tissues and organ systems found in more advanced animals.

4. Movement: While most sponges are sessile (attached to a surface) and appear immobile, some species can move very slowly. This ability for movement, even if limited, further differentiates them from plants.

5. Lack of Cell Walls: Plants have rigid cell walls composed of cellulose. Sponges lack cell walls, which allows for more flexibility in their body structure.

6. Presence of Specialized Cells: Sponges possess specialized cells that perform specific functions, such as filtering water, capturing food, and creating support structures. This level of cellular specialization is not found in plants.

In summary, sponges lack the defining characteristics of plants, such as chlorophyll for photosynthesis and cell walls. They are heterotrophic, multicellular organisms with some level of movement, and possess specialized cells, making them clearly classified as animals, albeit simple ones.