They don't last. The rain comes, storms, winds, other bathers who care little about your castle and they knock it down - forever lost and forgotten. It's a strange experience to build with or upon sand. Yet, people do it. Think of the millions of people who live along the Florida and Carolina coasts, they've built their holiday homes on sand reefs.
Builders have to dig down deep into the bedrock in order to firmly secure the structure.
Still, when the terrific Nor'easters go barreling up the coast, washing away the coastline, so too go the homes. Nothing can withstand the force of those storms, the winds, and the flood surge. Jesus tells us that when we listen to him it is like one who builds his house upon a rock. The rain fell and the flood came, the winds blew and beat upon the house and yet it did not fall. It had been built upon the rock.