While a lot of these answers are good, nobody has also pointed out one of the properties of water responsible for this. Aside from saturation in the sand, water molecules are cohesive—they like to stick to each other. This is due to hydrogen bonding in the molecules. Since water is “sticky” and likes to keep in touch, molecules can pull each other. This is why waves are able to pull the water back, as well as the reason (along with the adhesive nature of water, or the tendency to stick to other stuff) is why some water may run down the glass when you pour it.
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