You walk to the beach over sand that slides and pushes out from under your feet. If you pick some up, it runs through your fingers, almost as hard to contain as water. Theoretic It doesn't take long to learn that water mixed with sand changes both completely. Two fluids become a formable solid. It may be counterintuitive, but anyone who has spent time in the playground knows this in the fingers.
Water tends to pull in on itself.
This makes raindrops, rainbows and flying sparks from every sprinkler. Anything in contact with the water feels this pull, imparting a tiny tensile characteristic to damp sand. It sticks together. This adhesion is the essence of sand sculpture. It's also the bane, because damp sand stickily resists being compacted beyond a certain point no matter how hard it's hit.