Make a simple syrup:
In a small saucepan, combine the water and sugar.
Stir continuously while heating over medium heat until the sugar dissolves completely.
After the sugar dissolves, take the saucepan off the heat and allow it to cool to room temperature.
You can store any leftover simple syrup in an airtight container in the refrigerator for up to a month.
Prepare the lemon juice:
Squeeze the lemons to extract fresh lemon juice. If you prefer a less tart drink, you can strain the juice to remove any pulp.
Mix the lemon soda:
In a large pitcher, combine the fresh lemon juice and the simple syrup. Stir well.
Slowly pour in the cold sparkling water or club soda, stirring gently to combine. Be careful not to overstir, as this can cause the soda to lose its fizz.
Serve:
Fill glasses with ice cubes, and then pour the lemon soda over the ice.
Garnish with lemon slices and fresh mint leaves, if desired.
Serve immediately and enjoy your refreshing, non-alcoholic lemon soda!