In a medium saucepan, combine equal parts sugar and water. Heat on medium, stirring occasionally until it comes to a gentle simmer. Let it simmer for about 5 minutes until it thickens. Cool completely at room temperature, then refrigerate.

Slice 4 to 6 fresh limes in half and juice them into a bowl or measuring cup, aiming for about 1 cup of lime juice. Strain out any seeds or pulp.

In a cocktail glass, combine ⅛ cup of lime juice with an equal amount of syrup. Stir gently.

Fill the glass with cold sparkling water, stirring gently to combine while keeping the fizz intact.

Garnish with a lime slice, serve immediately over ice.