Generally sweet dessert wines are created when the grapes stop fermenting before all their sugar has turned to alcohol. This "residual" sugar is what makes these wines taste so full in your mouth and sweet. The fermentation either stops naturally or the winemaker stops it by the addition of sulphur dioxide. The best sweet wines are made from grapes that have super concentrated sugar. There are three main ways this is achieved. 1. The grapes can be dried in the sun. 2. They may be left in the frost (ice-wines). 3. They may be infected with noble rot caused by the fungus Botrytis cinerea.