Cut the top off the watermelon and hollow it out using a large spoon, fishing out any pips along the way. Mash the flesh through a sieve into a bowl. Put the flesh in a blender (or use a tall jug and hand blender) with the lemon juice and sugar, whizz to a purée, then stir in the soda.

Heap some ice into the hollowed-out watermelon and fill it with the lemonade mixture. Serve the rest in a jug with the lime slices and mint. Have a bowl of extra crushed ice on the side so people can help themselves.