Water has a substantial impact on the properties of most types of biological matter, but this impact is viewed as a perturbation rather than being foundational to the material. We recently introduced a theory that argues water gives biological matter its most basic properties (1). In this new picture, water is not a modifier of the physical characteristics, rather, the macroscopic physical characteristics, including the solidity of these materials, emerge from the physics of water, through the hydration force. The theory successfully explains qualitative and quantitative aspects of a rich set of known mechanical behaviors of these materials. And importantly, the theory helped discover novel equilibrium and non-equilibrium phenomena in solids, originating from the physics of water. After presenting the theory, I will show how it predicts the properties of many biological materials, with an emphasis on cellulose-based materials. I will present phenomena previously unrecognized in cellulose-based materials that the theory helped discover. Overall, the results make the case that these diverse materials constitute a major class of matter in Nature based on a common and highly distinctive underlying physics.