The time that a cat spends to fill a sink is about 5 seconds, and we observe this phenomenon for about 60 seconds. In this case, the relaxation time (5s) is smaller than the experimental time (60s), so the Deborah number is less than 1. Thus, under this circumstance, this cat is a liquid. 

1. When cats fit themselves into a container, they always alter their shapes to fill out the container without changing their volume. This is a property possessed by liquid. 

2. On an inclined surface, liquid tends to flow down due to gravity. In the picture presented below, it is evident that these cats are ready to “flow” down the stairs.

3. Liquids usually have a main direction in which they flow without the help of a container. The image below shows an example of a cat “flowing” along its main axis, which is the main direction of its movement. It is important to note that a liquid has a lower malleability when it is not moving along its main axis.

Therefore, it is fair to say that cats are a type of “non-Newtonian liquid” according to the proof above.