For the lemon syrup, peel 3 lemons with a potato peeler and set the peel aside.
Roll all 6 lemons across the countertop with the palm of your hand to make them easier to squeeze.
Cut the lemons in half and squeeze.
Place the sugar in a large measuring cup and pour 1.2 quarts of boiling water over it.
Stir until the sugar has dissolved, then mix in the citric acid and lemon juice.
Divide the strips of lemon peel among warm, sterilized bottles or cold-resistant containers and pour the lemon syrup over them using a funnel (if the syrup is to be frozen, fill the containers only 3/4 full).
Seal the containers airtight and store either in a cool place or in the freezer.
To drink, mix the syrup with soda water or with still water, or make pink lemonade from it.