In a saucepan, over low heat, mix the sugar and the 1/4 cup of water. Stir occasionally, until the sugar dissolves into a clear syrup. Over medium-high heat, cook the syrup until it turns a deep amber caramel. Every so often move the pan to swirl the sugar and distribute the heat, but do not stir. (see note)
With a sharp knife, slice 1/4 inch off the top and bottom of each grapefruit. With the grapefruit standing on its cut bottom, slice down the sides, following the fruit’s contour, completely removing the outside white pith as well as the peel. Hold the peeled fruit in your hand and remove the segments by cutting between the fruit and the membrane. Lift out each segment in one piece and drop into the caramel syrup. Cover and refrigerate at least two hours. The grapefruit sections can marinate in this caramel several days.
To prevent crystallization of the sugar, cover the pan with a stainless steel bowl; as the syrup cooks, water droplets will condense on the bottom of the bowl and wash down the sides of the pan.