What is the problem of the sandcastle?

Answer 1

Moisture will hold the sand grains together and a sandcastle can be built. When the sand dries out, the sand grains fall apart, and gravity and the wind will cause the sandcastle to become nothing more than a heap of sand. Or, as many children deliberately build their sandcastles between the low and high water mark, eventually, they have the delight of watching the incoming tide wipe away all their sandcastle building efforts.