Release the melon, remove the stone roughly. Peel and quarter the apple, remove the core and dice. Halve the lemon and squeeze out the juice with a lemon squeezer. Finely chop the kafir leaves. Pour all ingredients into a tall, slim container and puree them finely with a hand blender. Drain everything through a fine sieve and mix with the seltzer water (500 ml) and powdered sugar (2 tbsp). Fill into 4 glasses, add 2 ice cubes each and serve with a drinking straw.