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White, granulated sugar is the default for simple syrup, but you do have other options. Superfine sugar reduces the risk of crystallization. A raw brown sugar such as turbinado or demerara creates a brown, molasses-flavored syrup good for rum or bourbon cocktails. Do not use confectioner's sugar (powdered sugar). This usually contains cornstarch, which does not dissolve in water. The syrup will be cloudy or grainy. Measure the sugar and water and combine in a saucepan. Use an equal amount of both ingredients for a basic syrup. For a more concentrated syrup, use up to twice as much sugar as water.  A more concentrated syrup has a higher risk of crystallizing back into solids, but will last longer in the fridge. Some bartenders prefer a concentrated syrup because it can sweeten a cocktail without adding too much water. For greater accuracy, measure ingredients by weight on a kitchen scale. Using volume measurements (cups or milliliters) won't cause major issue, but you'll end up with about ⅞ the amount of sugar. Turn on the heat under your sugar-water mixture. Stir until all the sugar crystals have dissolved. The sugar usually dissolves within a few minutes, but a large batch may take longer.  Do not let the mixture boil. If you lose too much water, the sugar may not be able to dissolve. For very concentrated syrups (at least a 2:1 ratio of sugar to water), stir the last bits of water gently. Too much stirring when the maximum amount of sugar has dissolved can cause crystals to reform. A single grain of sugar left in the syrup can create a large mass of solid crystals. If you see any sugar left on the side of the pot, brush it down to the syrup using a wet pastry brush. Alternatively, just put the lid on the pot for a few minutes, and the condensed water should run down the sides and clean them. Because the lid traps most of the water vapor, it's all right if the syrup boils for a short time while the lid is on. To be safe, stick with a brief simmer. it will be ready to store once it reaches room temperature. If the sugar crystallizes when it cools, either too much water boiled away, or not all the sugar dissolved. Add a little water and heat it up again. Bring a small, separate pot of water to a boil. Once boiled, pour directly into a clean jar or bottle. Pour boiling water over the lid of the container as well. Sterilizing the container will reduce the chance that your syrup re-crystallizes, and prolong shelf life. Unless using immediately, store in a clear container so you can watch for signs of mold. Dump the hot water out of the container, and immediately pour in the room temperature syrup. Fasten the lid and store in the refrigerator.  A 1:1 syrup stays good for about one month. A 2:1 syrup stays good for about six months. To keep your syrup useful for much longer, stir in a spoonful of high-proof vodka.
Choose your sugar. Measure water and sugar. Heat and stir. Wash sugar off the side. Set aside the syrup to cool. Sterilize a container. Store the syrup.