The night before your picnic, puree 6 cups chopped seedless watermelon in a blender until smooth, pour into a shallow airtight container and freeze. Bring the frozen watermelon on your picnic (it'll double as an extra ice pack) and let thaw slightly, then mash with a fork and spoon into cups.

Source: https://www.foodnetwork.com/recipes/food-network-kitchens/watermelon-slushies-recipe/index.html

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