Always wear eye protection during science experiments. Never handle sharp objects, open flames, chemicals, or other dangerous objects without adult permission and supervision.
Fill your cup with the purified water. Your cup should be tall enough so that it's filled about half way.
Squeeze all the juice from the lemon into the cup. You'll need an adult to help you cut the lemon. Alternatively, if you're using lemon juice from a bottle, etc., you can substitute about 60 mL.
Add the sugar to the cup.
Add the ice cubes to your concoction. If you taste your concoction now, you'll notice that it's not quite as sweet as lemonade. But we'll fix this in a bit. (How do you think?)
Stir in the baking soda, and watch the bubbles form. This step not only creates carbon dioxide, but also neutralizes some of the lemon juice's citric acid.
Take a sip of your instant acid-base soda. What do you think?