Thoughts crystallize (if they really are physical things) into their peculiar geometry, analogous to phase changes and sudden precipitations from solution of a solid form. 

The flux of fresh impulses redissolves them, only for a new configuration to reemerge. Subsequent patterns contain permutations, simple or complex, of previous states embedded within them. A certain "stickiness" prevails: some notions become more deeply embedded than others.  

It is just possible then, that, like snowflakes, no two thoughts are exactly alike. 

(Note that the crystal growth of a snowflake is a route to crystallization distinct from, e.g., gas deposition or precipitation from solution).