Combine water and sugar in a medium saucepan.

Bring to a boil, then reduce heat to medium-low and simmer, without stirring, for 2 minutes.

Remove from heat. Stir in lemon juice and lemon zest.

Pour mixture into a flat, shallow plastic container or baking dish. Cool 30 minutes on countertop.

Cover and freeze for 1 1/2 hours. Remove from freezer and use a rubber spatula to scrape the outer edges, stirring the partially frozen pieces back into the mixture.Return to freezer and freezer for 1 hour more. Stir again.

Continue stirring every 30-45 minutes, until the mixture resembles finely crushed ice.

Scoop into glasses. Serve immediately.