Cut off one end of the watermelon. Throw end away.
With a spoon, carve out watermelon and place all the yummy insides in a blender. Process it until smooth!
Pour through a strainer into a bowl or pitcher. Toss any solids or seeds.
Take 1 cup of watermelon juice and pour into watermelon rind.
Mix in the lime juice, rum, simple syrup, and elderflower liqueur.
Add ice, and enjoy!
(Use reserve watermelon juice for another round)