Biologists are the people who study and research about most of the things that are related to living things, including plants, animals, bacteria etc.
Example of topics they study include "ecology", like how different living organisms relate and interact with each other, or "cell biology", with small topics like how protein synthesis works or what is DNA or RNA.
Area of scientific studies that usually fall outside biology includes "astronomy" the study of space, or" chemical bondings", like how does elements react with each other.
