The properties of a solid include its consistent volume and shape. A liquid, on the other hand, has a consistent volume, but takes on the shape of its container. A gas takes both the shape and volume of its container. As you can see from the image above, cats take the shape of their container, whether it's a box or a fish bowl, so therefore cats are liquid. It's been proven in an Ig Nobel prize-winning study.
But why are cats this way? What makes them so flexible that they can flow through fenceposts and floor cracks, and curl up in an impossibly small serving dish? It's all in their bones. A cat's skeleton, whether it's a house cat or a cheetah, has several anomalies that make it different from other mammals. Read about a cat's weirdly flexible skeleton at Inverse, and you'll wonder why other animal species haven't developed these keys to flexibility. Cats may be more alien than liquid. -via Damn Interesting