How do you soften stiff cardboard?

To soften stiff cardboard, you can lightly mist it with water using a spray bottle, ensuring not to soak it. After misting, gently fold and manipulate the cardboard to help it regain flexibility. Alternatively, placing it in a humid environment, such as a bathroom during a hot shower, can also help relax the fibers. Allow the cardboard to dry in its new shape to maintain the softness.