How to Build a Sand Castle?

Fine sand is better at holding water, meaning they form even better capillary bridges. In beaches where the grains were slightly more angular, they locked together better. When you go to the beach, just use your senses, see and feel how the sand is holding together. It's a discovery process. And it actually has real applications in civil engineering. Builders have to figure out the stability of the soil and see whether it can hold up to stress from the elements.