Here's why:

* Multicellularity: Animals are complex organisms made up of many cells that work together in specialized tissues and organs. Protists, on the other hand, are mostly single-celled organisms.

* Heterotrophy: While both animals and protists are heterotrophs (meaning they get their food by consuming other organisms), this isn't a distinguishing characteristic. Some protists are autotrophs (make their own food through photosynthesis).

* Movement: Both animals and protists can move, but this is not unique to animals. Some protists have flagella or cilia, which they use for movement.

* Eukaryotic cells: Both animals and protists are eukaryotes, meaning they have cells with membrane-bound organelles. This is not a characteristic that differentiates them.

In summary, the most fundamental difference between animals and protists is their level of organization. Animals are multicellular and highly organized, while protists are mostly single-celled.