Strain into a chilled highball or Collins glass filled with fresh ice.
Top with grapefruit soda and give a gentle stir to combine.
Garnish with a lime wedge or mint sprig and serve immediately.
Notes
Use fresh juices for best flavor. Skip the simple syrup for a drier drink or add a pinch of salt to make the citrus pop. Rim the glass with salt or sugar if you like, and keep ingredients cold for maximum fizz and minimal dilution.