1. Begin the preceding night by squeezing the lemons into a strainer formed by placing a coffee filter in a strainer. Strain the lemon juice into a small bowl containing the cold water - this is a very slow drop-by-drop process.
2. Add the sugar the next morning.
3. Mix very well with a wooden spoon until the sugar is completely dissolved.
4. Pour into a tin or aluminum pan.
5. Place the pan in the freezer.
6. Let rest for about 5 hours, breaking up the solid layer of ice which forms at least once every hour.
7. Remove the pan from the freezer.
8. Cut the ice into pieces with a knife.
9. Place the ice pieces in an ice crusher (or into a blender running at low speed).
10. The ice should form a smooth consistent texture of imperceptible grains (almost like ice cream - no large grains or chunks).
11. Transfer to individual serving glasses.