Here's why:

* Eukaryotic cells have a true nucleus, which is a membrane-bound organelle that contains the cell's genetic material (DNA).

* Prokaryotic cells (like bacteria and archaea) lack a nucleus and have their DNA located in a region called the nucleoid, which is not enclosed by a membrane.

So, any organism that is made up of cells with nuclei (like animals, plants, fungi, and protists) is classified as a eukaryote.