Add the chilled watermelon cubes to a tall glass and gently mash them with the back of a spoon to release their juice and create a light pulp base—you want to break down about half the pieces while keeping some texture intact. Add the fresh mint leaves and gently press them into the watermelon to release their aromatic oils. This creates a flavorful foundation that will infuse the entire drink.

Add the ice to the glass with the muddled watermelon and mint, then shake the Yakult bottle well to ensure the probiotics are evenly distributed throughout. Pour the Yakult slowly over the ice, allowing it to mix gently with the watermelon juice. Add the fresh lime juice and stir everything together until well combined. I find that adding the lime juice at this stage rather than at the beginning keeps it from getting lost in the mashing process—it adds brightness right when you're about to drink it.

Pour the drink into a serving glass if desired, or drink directly from the mixing glass while the ice is still cold and the flavors are bright. Serve right away before the ice begins to dilute the drink too much.