a device for piercing a nut has a cup - shaped member with a manually engageable , convex surface and a concave underside . a cutter is mounted on the concave underside of the member and points along a thrust direction . a retractable element is mounted adjacent to the cutter to reciprocate along the thrust direction . this retractable element can retract relative to the member to expose and give the cutter clearance for piercing the nut . the device is used by placing the retractable element over the nut with the cup - shaped member over the element . then the cup - shaped member is thrust down to pierce the nut with the cutter . when the cup - shaped member is released , the retractable element moves relative to the cup - shaped member to push the nut off the cutter .