Castles of sand
Question: Why does damp sand hold the shape of my castle-shaped bucket on the
beach whereas dry sand just crumbles away to nothing?
Answer: To understand this problem you need to consider two ideas: friction
and surface tension.
If you put a small block of wood on a table you can slide it around easily,
but if you put a heavy weight on top of the block you will need to apply a much
bigger force to move the block. This is a simple example of friction. The bigger
the force holding the block against the table,…
