A close-up view of ordinary sand.
Image credit: NASA
Grainy substances are so hard to figure out because they're so complex. In a heap of unmoving sand, for instance, each grain interacts with five to nine immediate neighbors all at once. The transitional state, when the heap begins to move, is scarcely easier: Although each grain is simultaneously interacting with maybe only three to five neighbors, those are not the same neighbors from one moment to the next. Even a supercomputer can't keep track of all the interactions.