Damp sand sticks together because water forms little grain-to-grain bridges. Surface tension–the same force that lets some insects walk on the surface of a pond–acts like rubberbands between the grains. Adding water to damp sand fills spaces between the grains. The bridges vanish and the sand begins to flow more easily.
Sand particles are quite small, so they have a relatively low volume/mass compared to their surface area (square-cube law). That’s important.
Water undergoes strong cohesive (which is responsible for surface tension) forces with itself due to hydrogen bonds, and it also has strong adhesive (sticking) forces as a result of induced interactions with silicon dioxide – also called wetting. These forces help the light sand particles to stick together, especially when you look at the scale where water is extremely effective for this purpose – as opposed to say ball bearings, pebbles, or golf balls.
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