Imagine you are explaining microorganisms to a friend who has never heard of them. How would you describe what they are and why they are often invisible?

Microorganisms are tiny living things, like bacteria, fungi, and protozoa, that are so small you can't see them with just your eyes. You need a microscope to view them because their individual cells are far too small for the naked eye to detect.

This question assesses the student's basic understanding of microorganisms and their characteristic of being invisible to the naked eye.