If you want a salt rim, rub a lime wedge around the rim of the glass and dip it in salt.
Put cucumber slices and mint leaves in the shaker.
Muddle gently to release their oils and flavors.
Add tequila, grapefruit juice, lime juice, and simple syrup to the shaker.
Fill the shaker with ice and shake for 10-15 seconds.
Strain the mixture into an ice-filled highball glass.
Pour in grapefruit soda and stir gently.
Add a cucumber slice and mint sprig as garnish.
Notes
You can use store-bought grapefruit juice, but fresh juice gives better flavor.Serve the drink right away while the bubbles are still fresh.For a spicy twist, muddle 2-3 thin jalapeño slices with the cucumber and mint.To make it non-alcoholic, use extra grapefruit soda instead of tequila.