In a saucepan, combine flour and sugar. Gradually add cold water; mix well. Cook and stir over low heat until mixture thickens to pudding consistency.
Place sand in a large pail; stir in flour mixture. When cool to the touch, mix together with your hands, adding more water if needed so that sand holds its shape. Firmly pack into molds. Invert on a flat surface; remove molds.
Let dry completely before handling. Depending on the humidity, this may take a few days.
To create a centerpiece, weave rope between the sand castles; fill in with sand, rocks and seashells.
Editor's Note: The sand mixture will keep for weeks when stored in an airtight container.