Such materials are said to deform visco-plastically. This implies that the material can plastically deform, and, therefore, is tough. The chewed bubble gum, on the other hand, will plastically deform enormously before finally breaking. In extreme cases, the ice can deform or distort mechanical elements, causing permanent damage. A ductile material will further deform after yielding. A rivet compression or crimping tool can also deform this type of rivet. Usually, attempting to deform bulk diamond crystal by tension or bending results in brittle fracture. Bullets are generally designed to penetrate, deform, or break apart. Less expensive woks have a higher tendency to deform and misshape. No truly rigid body exists; external forces can deform any solid. A higher modulus typically indicates that the material is harder to deform. In addition, surface forces can deform the droplet. Slowly they deform, until their internal rigidity is exceeded. Smaller balls or rollers deform more sharply, and so tend to fatigue faster. Solid objects will deform when adequate forces are applied to them.