[Free Registration/Login may be required to access all resource tools.] How do you define a living thing? What do mushrooms, daisies, cats, and bacteria have in common? All of these are living things, or organisms. It might seem hard to think of similarities among such different organisms, but they actually have many properties in common. Living organisms are similar to each other because all organisms evolved from the same common ancestor that lived billions of years ago. Learn more about characteristics of life in this learning module produced by CK-12.