French physicist Marc-Antoine Fardin was awarded an Ig Nobel Prize a couple of months ago for his groundbreaking research in rheology, which is the study of how matter flows and deforms. One of the problems in rheology is the definition of terms. The definition we learn in school is that a liquid is the state of matter that takes the shape of its container, but not the volume. The definition is further refined for scientists.
At the center of the definition of a liquid is an action: A material must be able to modify its form to fit within a container. The action must also have a characteristic duration. In rheology, this is called the relaxation time. Determining if something is liquid depends on whether it’s observed over a time period that’s shorter or longer than the relaxation time.
If we take cats as our example, the fact is that they can adapt their shape to their containers if we give them enough time. Cats are thus liquid if we give them the time to become liquid. In rheology, the state of a material is not really a fixed property—what must be measured is the relaxation time. What is its value, and on what does it depend? For example, does the relaxation time of a cat vary with its age? (In rheology, we speak of thixotropy.)
The whole point of the article that won the award, "On the Rheology of Cats," (found in this journal) is that if cats can fit into the scientific definition of a liquid, then maybe most of us don't know enough about the states of matter. Fardin gives us non-physicists a short course in rheology, specifically a breakdown of the ideas and terms that went into the paper at Slate.