Special equipment: an absinthe spoon and a lighter
Pour the absinthe into a rocks glass. Quickly soak the sugar cube in the absinthe; just roll it around to coat. Remove the sugar cube.
Place an absinthe spoon across the rim of the glass and set the soaked sugar cube atop the spoon. Light the sugar cube on fire and let the sugar caramelize for 10 to 20 seconds.
Slowly pour the water over the sugar cube, allowing the sugar to dissolve into glass.
Stir lightly with the absinthe spoon. Serve without ice, neat.