Put the watermelon chunks with sugar and mint leaves in a blender and blend until smooth. Strain through a fine strainer, pressing through well but holding back seeds and thick pulp. You should get around 4oz/120ml of juice.

Squeeze the juice from the lemon into watermelon juice and mix together with chia seeds(if you used), then add some ice. Top up the glass with soda water and stir before serving. Slap a sprig of mint in your hand to use as a garnish, as well as a slice of watermelon over the glass.