Two ruby grapefruit segments muddle in the shaker until they release their juice, then silver tequila, orange liqueur, agave nectar, and fresh lime juice go in over ice. The grapefruit adds a bitter edge that a standard lime margarita doesn't have — the agave softens it without masking it. Strain into a salt-rimmed glass and add one fresh grapefruit segment as a garnish. Single serving, five minutes.