In each glass muddle the juice of half a lime and 3-4 torn mint leaves.
OPTIONAL: if you prefer a spiked drink add 1 -1.5 oz of vodka or rum to the glass when muddling the lime and mint.
In a blender combine ice cubes, watermelon, and frozen strawberries until a thick slush is made. To assist blending you may need to use a spoon or spatula to push the contents of the blender closer to the blade.
Pour approximately 1 1/2 cups of sparkling water in each glass. Use lemon lime soda if you prefer a sweeter drink.
Scoop slush into each glass until it is full to the brim. Serve immediately!