Make a simple syrup with 1 cup of water and 1/2 cup of sugar. This is done by bringing the sugar and water to a simmer over medium heat until the sugar dissolves and the water is clear. Remove from heat and allow to cool completely. You can do this by covering it in cling film and placing it in the fridge or freezer.
Wash limes thoroughly. Cut off the ends and slice into eight wedges (including rinds). Place limes in an electric blender with remaining 2 cups of water and pulse til they're pureed. Strain through a very fine mesh strainer keeping only the juice from the limes.
Pour the lime juice back into the blender and add the cold simple syrup, sweetened condensed milk, and ice. Blend again to mix and break up the ice.
You can make it alcoholic by substituting 1 cup water with an equal portion of vodka or rum.
Note: this doesn't keep well overnight.
