Optionally, rim a highball glass with salt by running a lime wedge around the edge and dipping it into a saucer of salt. Fill the glass with ice cubes.
Using the jigger, measure and pour blanco tequila, fresh lime juice, and agave syrup into the glass.
Top with grapefruit soda.
Use a bar spoon to gently stir the mixture.
Garnish with a lime wedge or grapefruit slice.
Notes
For a smoky twist, substitute the tequila with mezcal.