Cut 1 kg of watermelon into small cubes, removing the seeds and rind. Place the watermelon cubes in a blender.
Add 50 grams of sugar and the juice of 1 lemon to the blender. Blend the mixture until smooth and well combined.
Pour the blended watermelon mixture into a shallow, freezer-safe container. Cover the container with a lid or plastic wrap.
Place the container in the freezer for about 2 hours. After 2 hours, take the container out and use a fork to scrape and break up the partially frozen mixture.
Return the container to the freezer and repeat the scraping process every 2 hours for a total of 6-8 hours, or until the granita reaches a fluffy, snow-like consistency.
Once the granita is ready, scoop it into serving bowls or glasses and serve immediately.