Here's why:

* Eukaryotes are organisms whose cells have a true nucleus, which contains their genetic material (DNA).

* Prokaryotes, on the other hand, are single-celled organisms that lack a nucleus and other membrane-bound organelles. Their DNA is located in a region called the nucleoid, but it's not enclosed within a membrane.

So, any single-celled organism you find that has a nucleus is classified as a eukaryote. Examples include:

* Protozoa (like amoebas and paramecia)

* Algae (like diatoms and seaweed)

* Yeasts (a type of fungi)