Here's why:

* Plants are multicellular organisms that produce their own food through photosynthesis. Grass is a common type of plant.

* Animals are multicellular organisms that consume other organisms for food.

* Algae are mostly single-celled organisms that can photosynthesize. While some are large and multicellular, they are not plants.

* Fungus are multicellular organisms that decompose organic matter. They don't photosynthesize.