Summary
Ig Nobel Prize proved this. Cats take the shape of their container - that"s literally the definition of liquid.
A physicist won the 2017 Ig Nobel Prize for scientifically proving cats are "both liquid and solid."
Definition of liquid: matter that takes the shape of its container. Cats can fit in boxes, vases, sinks, even glasses. Their flexible skeleton and floating collarbone allow this.
Cats have 230 bones - humans have only 206. Extra bones are in the spine, providing incredible flexibility.
Source: Rheology Bulletin, Ig Nobel Prize