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If you want your cherries on the super-tart side, add less sugar. You can also use a sugar substitute (like Splenda), honey, or agave syrup. To get the cherry pits out, score them along their sides with a knife. You may be able to pop the pits right out -- or just take a butter knife and wedge them out if they're being difficult. The cherries need time to soak up the sweetness of the sugar. It'll be so tangy that you'll need the water to dilute it later. You're looking for one mostly uniform consistency (the cherry chunks will keep it from being entirely uniform). Then reduce the heat and simmer for fifteen minutes. It should maintain a light bubble the entire time, reducing the liquid and turning almost syrupy. Squeeze all the moisture from the cherries into a separate saucepan. Don't just strain it -- really squeeze out all the juices. You're finished with the cherries; you can throw them away. Or keep them for a topping or a preserve! Then take the pan off, let it cool to room temperature and move it to a sealed vessel for storage in a refrigerator. That's it! The consistency is right; this is basically tart cherry juice concentrate. It's not supposed to be like juice -- it should be much thicker. Sparkling water (or even just water) works, too. Feel free to experiment with the ratio to find your personal taste. It may take a try or two -- but once you find the right combination, it'll be easy. Keep the rest in a resealable container to save for later. It'll keep for a couple weeks if kept tight and in the refrigerator.

Summary:
Put the clean, seedless cherries and sugar in a saucepan. Cover the pan and leave at room temperature for 2 hours. Add the 1/2 US-pint (950ml) of water and stir until all the sugar dissolves. Bring the contents up to a boil. When finished, strain the mixture. Simmer the strained liquid until it thickens like maple syrup. To serve this drink, put one or two spoons of it into a glass of soda water.