You could dip the eraser in water for a few seconds, or you can use a damp cloth or sponge to moisten the eraser's surface. Afterward, knead the eraser again to distribute the moisture throughout. This can help rehydrate the eraser and restore its elasticity. If the eraser is extremely hard or has become a block of rubber, you may need to trim away the hardened parts. Use a craft knife or scissors to carefully cut away the dry sections, reshaping the eraser into a usable form.