Here's why:

* Mold is a type of fungus.

* Fungi are eukaryotic organisms, meaning their cells have a nucleus and other organelles.

* Mold consists of many interconnected fungal cells, forming long, thread-like structures called hyphae.

* Hyphae intertwine to form a network called a mycelium.

Therefore, the mold that grows on bread is a multicellular organism.