"Cats are liquid" is a common humorous saying used to describe cats' bodies being very pliable and flexible, able to adapt to a variety of shapes and spaces. This is because cats' skeletal structure and muscular system allow them to perform some surprising actions, such as squeezing into small spaces, bending their bodies, stretching their limbs, and more.

The softness of cats' bodies allows them to maneuver through narrow passages and squeeze into boxes or other small spaces. Additionally, cats have very flexible spines, which allows them to bend and twist their bodies to accommodate different postures and movements.

This statement mainly emphasizes the physical adaptability and flexibility of cats, not that cats are really liquid. It is an image expression used to describe the unique characteristics of cats. Of course, every cat's personality and behavior are different, and some cats may be more flexible than others.