1 large or 2 small watermelons
8 oz lemon juice (from a bottle or squeeze your own)
1/2 cup sugar (super-fine if you have it)
1l bottle soda water
1 lime , cut into slices
small handful mint
STEP 1 Cut the top off the watermelon and hollow it out using a large spoon, fishing out any seeds. Mash the flesh through a sieve into a bowl. Put the flesh in a blender with the lemon juice and sugar, blend to a purée, then stir in the soda water.
STEP 2 Pour mixture into a highball glass over ice and garnished with lime slices and mint. Alternatively, heap some ice into the hollowed-out watermelon and fill it with the lemonade mixture. Serve the rest in a pitcher with the lime slices and mint.
Looking for a boozy version? Add vodka, tequila, or white rum.