You need to understand pottery clay properties to understand pottery clay. While the clay you pull from your garden is crumbly, molding clay is slick and sticks together while holding it's shape. Molding clay does this because of plasticity.  Plasticity allows the clay to be formed, shaped, and molded so you can make sculptures and pottery.  Clay is made up of decomposed feldspathic rocks, rocks like granite. That granite washes down mountains and settles in creek bed and lakes to become sediment.  This sediment is how clay is formed.As erosion occurs more and more clay is deposited and formed.  Particles of clay are shaped like small, flat platelets similar to flecks of mica. If you look at a clay particle under an electron microscope, it looks like flat pieces stacked on top of each other.  While they are difficult to separate they slide across one another easily.  This is the principle occurring in clay that allows it to expand and contract, be manipulated, and still hold its shape.