To make the simple syrup put the key lime juice and sugar in a saucepan.

Bring to a boil over medium heat, stirring frequently, until sugar has dissolved.

Remove from heat and cool completely.

When ready to serve rim a glass with sugar or salt. Fill with ice.

Combine 3 ounces of the key lime syrup with the tequila and orange liqueur in a cocktail shaker and shake to combine. Pour over top of the ice in the glass.

Fill the glass with bubbly water.

Place a key lime slice on the rim and serve with a straw.