In a small saucepan over medium heat, combine the water and sugar, and boil until the sugar dissolves and it starts to thicken, about 3 minutes.
Remove from heat and allow the syrup to completely cool.
Once cool, stir in the lime juice and lime zest.
Pour the mixture into a 9x5 metal baking pan.
Freeze until firm (about 4 hours), stirring with a fork every thirty minutes for the first 3 hours. This is important. Don’t skip this step if you aren’t using an ice cream machine.
Store covered in the freezer for up to 3 months.
