Simple Syrup:

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  In a saucepan, combine the sugar and water.
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  Heat the mixture over medium-high heat, stirring occasionally until the sugar is completely dissolved.
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  Once the sugar has dissolved, remove the saucepan from the heat and let it cool to room temperature.
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  Once the mixture has cooled, transfer it to a clean bottle or jar, and store it in the refrigerator until you're ready to use it.
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  Blend the cubed watermelon, sliced cucumber, and fresh mint leaves in a blender until smooth.
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  Pour the mixture into a large pitcher, then add the vodka, lime juice, and simple syrup. Stir well to combine.
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  Fill the pitcher with ice cubes and top it off with the club soda.
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  Give the mixture a gentle stir and serve immediately.