As Fardin pointed out in his paper, cats share other properties with liquids. For example, they have a yield stress, meaning that a minimum amount of force must be applied before they flow out of a container. The same applies to ketchup in a plastic bottle, which must be squeezed out. In addition, cats, like a fluid, adapt their body to the vessel they enter so that they fill it completely. Another characteristic that cats share with some liquids is their high surface tension, which comes into play as they press in or out of a small container.

Fardin was also interested in other flow properties of cats, such as whether they could create turbulence. But cats, he noted in his paper, fall in a class of “biologically active materials” alongside bacteria, flocks and schools, which have “their own motive power,” and are therefore hard to assess in this way. “In conclusion, much more work remains ahead, but cats are proving to be a rich model system for rheological research,” Fardin wrote.

Biology offers a different lens on this question. From a life science point of view, these animals resemble a liquid—much more so than other creatures such as humans—because of their movable (and sometimes missing) collarbone. Once their head fits through an opening, the rest of the body can easily follow. This is how Tigrou flowed into the narrow gap beneath our fireplace.