Make the mint syrup: In a small saucepan, combine the sugar and water. Bring to a gentle simmer over medium heat, stirring occasionally. After about 3 minutes, the sugar will dissolve completely and the liquid will turn clear. Add the mint leaves, crushing them slightly between your fingers to release their oils. Turn off the heat, cover the pan, and let it steep for 20 minutes. The syrup will take on a pale green hue and smell wonderfully herbaceous.

Prepare the watermelon: While the syrup steeps, place the watermelon chunks into a blender. Blitz on high for about 30 seconds until the flesh breaks down into a deep pink, frothy liquid. You should see tiny seeds floating—don’t worry about them yet. The sound will shift from a chunky thud to a smooth whir.

Strain the juice: Set a fine-mesh sieve over a large bowl or jug. Pour the blended watermelon through it, pressing gently on the solids with the back of a spoon to extract every drop of juice. Discard the pulp and seeds. You should have about 400 ml of clear, vibrant juice. The colour should be a brilliant ruby red.

Finish the syrup: After 20 minutes, remove the mint leaves from the syrup, squeezing them over the pan to extract any remaining flavour. Stir in the lime juice. The syrup will now be a pale greenish-gold, with a sharp, sweet scent.

Combine the base: In a large jug, mix the watermelon juice with the cooled mint syrup. Stir gently until evenly combined. Taste it—it should be sweet and tangy, with a strong mint finish. If you like, add a tiny pinch of sea salt to lift the flavours.

Chill thoroughly: Place the jug in the fridge for at least 45 minutes. This step is crucial; a warm cooler is a flat cooler. The mixture should feel icy cold to the touch when you take it out.

Serve with sparkle: Divide the chilled watermelon-mint base among four tall glasses filled with ice cubes. Top each glass with about 150 ml of sparkling water, pouring slowly to preserve the fizz. Stir once gently with a long spoon. Garnish each glass with a fresh mint sprig and a thin slice of lime. The bubbles should dance up through the red liquid, releasing a fresh, sweet aroma.