If using a food processor, place the flour, sugar, salt, and baking soda in the processor. With the blade spinning, slowly pour the lukewarm water through the feed tube until a ball of dough forms. Continue to process for 15 or 20 seconds. If working by hand, combine the flour, sugar, salt, and baking soda in a large bowl. Stir to mix. Make a well in the center and add 1 cup of the water. With your hand or a wooden spoon, stir the water into the flour mixture until all the flour is moistened; add more water if necessary. Knead in the bowl until you have a dough.

Turn the dough out onto a lightly floured surface, then knead it for several minutes, until it is very smooth and elastic. Cover with plastic wrap and let stand for an hour or so. The dough will soften and expand a little as it rests.

Lightly flour a work surface. Lightly oil a nearby surface.

Using a dough scraper or a large sharp knife, cut the dough in half. Set half aside, loosely covered. Working on the floured surface, cut the remaining dough into 8 equal pieces, cutting it in half and then in half and half again (each piece will weigh about 1¾ ounces). Roll one piece under your flattened palms, pressing outward slightly as you do so, into a long rope. The rope will gradually lengthen under your hands; once it is about 10 inches long, set it aside for a moment and start to roll and stretch a second piece in the same way. Come back to the first piece (the dough will have had time to relax a little) and roll it until it is 18 to 20 inches long.

Move the long rope onto the oiled surface. Anchor one end of it on the work surface with one thumb and roll the length of it under the palm of your other hand, rolling always in the same direction, so that gradually it twists more and more. When it is well twisted, pick up one end of the rope in each hand and bring the ends toward each other. The loop of rope will coil on itself. Pull on the bottom loop of the coiled dough to lengthen it slightly, then lightly press it flat on the oiled work surface. Once again anchor one end of it, the one with the two ends of dough, and use your other palm to roll the dough in the same direction as the coils that are already there, in order to twist it further. Lift up both ends and bring them toward each other. The loop will again coil on itself, slowly, making about one and a half twists. Bring the ends together and tuck the open end into the loop of the other end, then pinch to secure it.

The double-twisted dough will be about 4 inches long. Place it back on the oiled surface and press down lightly. Set aside on an oiled baking sheet while you shape the remaining 7 pieces, then divide and shape the other half of dough into another 8 twists (see Note on Timing). Set the sheet of shaped twists near your stovetop.

Place a large wok or deep heavy pot on your stovetop; make sure the wok or pot is stable. (Or use a deep-fryer.) Pour 2 inches of oil into the wok or pot and heat the oil over high heat. Set out a slotted spoon or mesh skimmer. To test the temperature of the oil, hold a wooden chopstick vertically in the oil, touching the bottom of the pan. If the oil bubbles up around it, it is at temperature. If it really sizzles or starts to smoke, the oil is too hot; lower the heat slightly and test it again. (A deep-fry thermometer should read 325°F to 350°F.)

Slide in one shaped twist. It will sink, then slowly rise back up. After about 30 seconds, use the slotted spoon or skimmer to gently turn it over. It should be golden, not dark brown. (If the doughnut darkens more quickly, your oil is too hot, so lower the heat slightly; see Note on Troubleshooting.) Let cook for another 30 seconds, then turn it back over. The second side should now be a medium brown. Cook for another 20 seconds or so on each side, or until the doughnut is medium to dark brown. You will see attractive pale lines where the twists cross over. Use the slotted spoon or skimmer to lift the twist out onto a paper-towellined plate.

Repeat, this time starting with one twist and then adding a second if your pot is large enough, when you turn it over for the first time. Cooking times will be a little longer when you cook 2 or more at a time. Cook the remaining shaped twists in the same way. Allow to cool for several minutes before serving.