Pour one measure (25ml) of absinthe into a glass.
Rest a perforated absinthe spoon on the glass.
Place one sugar lump on the spoon.
Take very fresh water and, drop by drop, drip it on the sugar cube so it dissolves and disintegrates, dropping into the absinthe with the water.
With every drip, the translucent green liquid begins to turn from a transparent green liqueur into a milky white-ish colour. This is called the louche effect and, for the likes of Lautrec and Verlaine, it was an integral part of the absinthe drinking ritual.