Place 2 (2-ounce) shot glasses in the freezer at least 15 minutes before you begin.
In a cocktail shaker filled with ice, add vodka, triple sec, lemon juice and simple syrup. Shake well, 15-20 seconds.
Strain into chilled shot glasses. Serve immediately.
Notes
Shortcut version: Use 1½ ounces sour mix instead of the lemon juice and simple syrup.To make simple syrup: In a medium saucepan over medium heat, combine 1 cup sugar and 1 cup water. Stir until sugar has dissolved. Remove from heat. Let cool completely. Transfer to a glass jar and seal tightly with a lid. Store in the refrigerator for up to 1 month.