Next, add the watermelon, lemon juice, and calcium mixture to a wide, deep pot. Bring to a boil. Stir occasionally. Add in the sugar mixture, stir, and boil for about 2 minutes. NOTE: Keep a close eye on the pot; temperatures can take a while to heat up, but then can change very quickly. Also, the watermelon will foam somewhat. Stirring will help keep that at bay.