Mix almost boiling water and sugar together until sugar is dissolved. Cool then refrigerate until cold.
Wash limes with soap and water rinsing very well. Cut the ends off limes and cut each into 8 pieces.
Place half of the limes and chilled sugar water into a blender and pulse 5-10 times. Pour through a mesh strainer into a pitcher. Repeat with balance of limes and sugar water. Stir sweetened condensed milk into the pitcher. Chill or serve immediately over lots of ice.
Optional: add rum or vodka for an adult version.