Combine the grapefruit juice, lime juice, and sugar in a glass; stir until the sugar is dissolved. Stir in the tequila, add a few ice cubes, and top it off with club soda. I like Topo Chico for an intensely carbonated but light taste, or Fresca grapefruit soda when I'm wanting something milder and sweeter.

Garnish the drink with a grapefruit wedge if you want to be fancy. If you want to be extra fancy and have a salted rim on your drink, before you begin you can rub the wedge around the rim of your glass and dip it in kosher salt.