Cut the nori in half lengthwise. Place one half on the rolling mat. Wet fingers and palms with rice vinegar and place about 1/2 cup rice on nori, spreading it evenly and flat, and leaving a blank strip at the top of the nori about 1/2-inch wide. (Optional: Smear a line of wasabi evenly across the rice a little below the center.) Place fillings of choice evenly across the rice a little below the center (or on the line of wasabi). Lift the end of the mat closest to you, and press down on the rolling mat to stabilize. Roll the mat almost one complete turn, making sure it does not become part of the roll and that the nori goes under the rice. Roll in the shape of a cylinder.
Cut each roll into 4 or 6 equal slices, dampening the cutting knife between each slice by running it across a cloth dampened with rice vinegar.
Rinse rice in water until the water runs clear and then drain in a colander for 1 hour. Place the drained rice in a rice cooker or in a pot with a tight-fitting lid and add 4 cups water. Over medium heat, cover and bring the water to a boil. Boil for about 2 minutes, reduce heat and allow to simmer for another 5 minutes. Reduce heat to low and cook for about 15 minutes, or until water has been absorbed. Remove from the heat, remove lid, and place a towel over pot. Replace lid and let stand for 10 to 15 minutes.
While the rice cooks, combine vinegar, sugar and remaining 1/4 cup water in a saucepan. Heat over low temperatures, stirring, until sugar and salt dissolve. Let cool.
Empty rice into a hangiri (or other nonmetallic) tub and spread it evenly over the bottom with a shamoji (or large wooden spoon). Run the spatula through the rice in slicing motions to separate the grains. While doing this, slowly add vinegar mixture. Add only as much as is necessary; the rice should not be mushy. If you have help, fan the rice with an uchiwa (fan) during the cooling and mixing procedures.
Do not refrigerate the rice. Keep it in the tub covered with a clean cloth until ready to use. The rice will last one day.
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