Pour a handful of dry sand: it will spread into a neat cone. Now grab a box of star-shaped pasta: the pile will stand almost vertical. Why? It's all about particle shape.
Physicists built cylindrical piles from particles of different geometries — from smooth spheres to 'polypods' with several arms, like a sea star. Each pile was scanned with an X-ray tomograph to see the position, orientation, and contacts of every particle. It turned out that for particles with protrusions, neighbors interlock like fingers of clasped hands, preventing each other from moving. Even without glue or friction, such a pile holds its shape.
The scientists derived a simple parameter: it accounts for how many contacts each particle has and how they are arranged. This parameter accurately predicted whether a pile would stand or collapse. This means you can calculate in advance which particle shape to choose so that an embankment, a pill, or a construction mix is as strong as possible.
In nature, sand wasps use a similar trick: they build nests from sand grains with sharp edges, and the nest holds together without any cement.