Mix the orange and lime juice with the sugar in a small saucepan and bring to the boil. Reduce the heat and simmer gently for 15 minutes until a thick syrup is formed.
Pour into a small bottle and store in the refrigerator after cooling.
Pour 2 tablespoons of syrup into a glass to taste and pour chilled sparkling water on top. A fruity and delicious lemonade is ready.