Place watermelon chunks in blender and blend briefly to coarse puree. Measure sieved puree; it should have yielded 3 cups.
Combine sugar and water in a medium saucepan and bring to a boil. Lower heat and simmer 1 minute. Let cool completely and combine with watermelon puree and lemon juice. Pour mixture in 8x12 metal pan and place in refrigerator. When completely chilled, cover pan, move to freezer and freeze 30 minutes. Stir mixture with fork, scraping down sides well to loosen ice crystals that have formed. Freeze another 30 minutes and repeat scraping and stirring. Repeat several times, or for a total of 23 hours, until mixture ice evenly icy and crystalled throughout. Serve within 68 hours.
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