1. Rub the rims of two cocktail glasses with the lime or lemon. Pour some salt into a saucer and dip in the glasses so that the rims are frosted.
2. Combine the tequila, Triple Sec or Cointreau, and lime and lemon juice in a jug and stir to mix well.
Pour the tequila mixture into the prepared glasses. Add the ice cubes and serve at once.
It really is worth going to the trouble of buying limes for this recipe. Lemons will do, but something of the special flavour of the drink will be lost in the substitution.