Peel the outermost green peel off of your leftover watermelon rinds and cut them into ¼-inch thick slices with a knife or a mandoline.

Place the rinds in a shallow container and pour the sugar over the watermelon rind slices, then stir to coat. Cover and refrigerate for a minimum of 6 hours, or overnight.

The watermelon will release a lot of water. Transfer the watermelon rind and all of the liquid into a medium-sized saucepan over medium heat.

Add 1 cup of water to the saucepan. Bring the mixture to a boil, stirring occasionally. The water will boil off for about 15-20 minutes or so, and in the process the watermelon will become glassy and translucent.

If you're using a candy thermometer (which we strongly suggest), you'll notice the temperature will start to climb into the 220˚F range. Keep cooking until the temperature reaches 235˚F - 240˚F, about 10-12 minutes, then move to a wider, shallower pan over low heat.

The goal now is to dry out the sugar without burning it, so stir frequently and keep a close eye on the mixture. If you start to get a burnt sugar smell or golden brown color, remove from heat immediately, stir, and then return to low heat in a minute or two.

Keep stirring until the sugar starts to dry out and crystallize on the watermelon rind. Once you see the sugar has mostly crystallized, add 1 tablespoon of citric acid to the candied rind and stir to coat.

At this point, there should not be any remaining liquid in the pan. Transfer the candy out onto a wire baking rack set atop a sheet pan, and use tongs to unfurl any pieces that have curled up during the cooking process.

Optional: Sprinkle on more citric acid, depending on how sour you like your candy. Keep stored in an airtight container at room temperature for up to a month, or longer in the refrigerator or freezer.