Zest the limes: Finely zest the limes, taking only the green outer skin and avoiding the white pith underneath. Don't press too hard when zesting, as the pith is bitter.

Mix the zest and sugar: Add the lime zest and sugar to a small saucepan and stir well. Let them sit for a few minutes.

Add the water: Pour the water into the saucepan and stir to combine.

Add the lime juice and simmer: Squeeze the fresh lime juice into the saucepan. Bring the mixture to a gentle simmer and cook until the sugar has completely dissolved.

Strain and cool: Remove from the heat and strain out the lime zest.

Fill the glass with ice: Add plenty of ice to a glass so the drink stays cold.

Add the lime syrup: Pour in 1–2 tablespoons of lime syrup, depending on how sweet and limey you like your drink.

Add the sparkling water: Slowly pour in very cold sparkling water. Pour it gently to help keep as much fizz as possible.

Stir and garnish: Give the drink a quick, gentle stir. Add a lime slice and a few fresh mint leaves (optional), and serve immediately.