In a glass jug, add your simple syrup and lemon juice and whisk together. Add your water and whisk until combined.
Pour into glasses filled with ice, and refrigerate leftovers.
* You can use seltzer (or soda water), but you cannot keep it for any longer than 24 hours. TO STORE: Place the prepared lemonade (with water) into a glass jar or bottle and store it in the refrigerator for up to 2 weeks. TO SERVE: Add 2-3 ice cubes into a glass and fill with homemade lemonade.