Ideal nigiri sushi has just the right amount of space between the grains of rice. If the topping (neta) is made from a delicate fish, the rice ball should be made a little firmer to prevent it from coming apart. Conversely, looser rice is used for firmer fish. Each piece should crumble apart in your mouth but not come apart when being picked up with chopsticks. A well crafted piece of nigiri should create the image of a live fish swimming away on the plate.