Fill a tall glass (or any 10 to 14-ounce glass) halfway with ice.
Add the tequila and lime juice.
Top with Topo Chico, give it a stir, and garnish with a lime wedge.
Notes
Tequila: Blanco (silver) tequila is traditional in this drink, but you can use reposado or even mezcal for a slightly smoother, smokier flavor. Always choose a tequila labeled 100% agave for the best taste. Espolon, Casamigos, Herradura, or Patron are all great brands.
Mineral water:Topo Chico is the top choice for its sharp carbonation and minerality. If you can't find it, use another sparkling mineral water like San Pellegrino or Perrier.
Lime juice: Fresh is best, but if you're using bottled, pick a high-quality pure lime juice that isn't from concentrate, like Santa Cruz Organic.