The herbal ingredient called wormwood (Artemisia absinthium) is very bitter, so the liqueur needed sweetening in order to be palatable. Sugar does not readily mix with alcohol, so the absinthe ritual was born.
A sugar cube was placed on a slotted spoon over the glass containing a shot of clear peridot green absinthe. When icy water was drizzled through the sugar to melt it, it begins to "louche." That is, the clear liqueur becomes cloudy and palest jade in color. This signals it is ready to drink. My main character in the book calls this ritual "watching the clouds come out."