How to Make It
Using a small paring knife, cut 1 end off watermelon in a zigzag pattern. Discard end. Using a spoon, remove watermelon flesh, and place watermelon flesh in a blender. Process until smooth. Pour through a fine wire-mesh strainer, and discard solids.
Return 1 cup of the watermelon juice to watermelon rind; reserve remaining watermelon juice for another use. Add rum, lime juice, simple syrup, and elderflower liqueur to watermelon; stir to combine. Top with ice, and share with 3 friends.