Why is a toadstool not a plant?

A toadstool is not a plant because it belongs to the kingdom Fungi, which is distinct from the kingdom Plantae. Unlike plants, toadstools do not perform photosynthesis; they obtain nutrients by decomposing organic matter or forming symbiotic relationships with other organisms. Additionally, toadstools have a different cellular structure, including chitin in their cell walls, whereas plants have cellulose. This fundamental biological difference classifies toadstools as fungi, not plants.