Cut the watermelon in half and scoop out the flesh, setting aside the flesh and separating it from the rind. Chop the flesh into smaller pieces for blending.
Using a sharp knife and cutting board, carefully peel the green rind from the watermelon, then chop the rind into small cubes. Set aside the chopped rind.
Place the watermelon flesh and chopped rind into the blender jar, add lemon juice and a pinch of salt. Blend until thoroughly combined and smooth, about 30-45 seconds, creating a vibrant, slightly pulpy mixture.
Pour the blended mixture through a fine mesh strainer or cheesecloth into a large bowl, pressing gently to extract the juice while leaving pulp behind. Discard the solids.
Transfer the strained juice into a pitcher, then chill in the refrigerator for at least 10 minutes until cold and airy. Observe the juice become clearer and more vibrant as it cools.
Serve the watermelon rind juice in glasses, garnished optionally with small watermelon rind slices or a twist of lemon. Note the translucent, lightly cloudy appearance and the refreshing aroma with citrus notes.
