Remove the elderberries from their stems and wash them thoroughly. Place the berries in a pot with the water and bring to a boil. Let the mixture boil well so the elderberries become soft.

Use a whisk to stir and mash the elderberries until they are very soft and release their juice. Strain the mixture through a fine mesh sieve, pressing to extract as much liquid as possible and discard the solids. Pour the strained juice back into the pot, add the rock sugar, and heat until the sugar dissolves. The amount of sugar here gives a lightly sweet, refreshing taste—add more sugar if you prefer it sweeter.

Once the juice has cooled, pour it into clean bottles and store in the refrigerator. Drink daily instead of water to help boost your immune system during cold and flu season.