Open a bottle of Topo Chico and dump about ½ cup worth of water into a glass. Save it for later when you want a refill! NOTE: keep all the lime, tequila, grapefruit, and Topo Chico cold before mixing for the best results!
Using a citrus juicer, juice two limes. Reserve a slice or two for your cocktail.
Pour the lime juice and grapefruit juice directly into Topo Chico bottle.
Eyeball or use a shot glass to add the tequila. I prefer white tequila than mezcal.
Using a reserved lime slice, rup the rim and next with it, and add the tajin (about a tablespoon) to adhear to the bottle.