Why shaking cannot free every grain
A towel is not a smooth sheet. Its loops create small pockets, and the spaces between woven threads form narrow channels. Sand grains that land in these places can wedge against several fibers at once. Friction—the resistance between surfaces pressed together—helps keep them there, especially when a grain is too large to pass easily through the gap it entered.
A quick shake moves the cloth, but it does not pull every grain straight out of its pocket. Some grains shift and settle deeper; others simply stay put because the fabric’s motion is not enough to overcome the friction and the surrounding fibers.
Water changes the hiding places
Damp fabric can hold grains in place in another way. Thin films of water between sand and fibers create surface tension, the same effect that lets water cling to small surfaces. As the fabric dries, that grip weakens, but the sand remains lodged in the weave. Moving or folding the cloth later can dislodge it.
Bags add more places for grains to collect: seams, zipper tracks, and folded corners can trap particles where a casual shake barely reaches. A few grains then migrate out when the bag is opened or its contents shift. The sand is not multiplying; it is being released in small batches from pockets the first shake never emptied.
