Make the lemon/lime simple syrup:
Add the cup of sugar to a small saucepan. Zest the lemons and limes right over the saucepan. Slice and juice the lemons and limes. You should have about 1 cup of juice. Add this to the saucepan, along with 1 cup of cold water. Bring the mixture to a boil over high heat. Lower the heat to medium low, then simmer until the sugar has completely incorporated and the mixture takes on a syrupy consistency, about 10-15 minutes. Remove the syrup from heat and allow to cool.
Note: This simple syrup is not as sweet as other fruit based syrups. In fact, it's very tart. If you prefer a sweeter syrup, reduce the lemons or add more sugar. Personally, I loved the tartness of this syrup.
Makes about 1 1/2 cups of lemon lime syrup
Make the cocktail:
To a cocktail shaker add the lemon/lime syrup, sugar, rum, and ice. Shake for about 10 seconds and pour into a glass. Top with seltzer and garnish with limes.
Knock it back, call your grandpa. Tell him how much you love him.
Note: If you don't own a cocktail shaker, add lemon/lime syrup, sugar, and rum to a glass. Give it a good stir until incorporated. Add ice, then top with seltzer.
Makes 1 cocktail. Double or triple if you wish.