A snowflake, at its core, is just water molecules that bind together into a particular solid configuration. But when Guinness certifies two snowflakes as identical, they can only mean that it's identical to the precision of the microscope; when physics demands that two things be identical, they mean identical down to the subatomic particle! When frozen water molecules bind together, each molecule gets four other water molecules bound near it: one at each of the tetrahedral vertices centered on each individual molecule. On the contrary, the remaining sides have their molecules much more exposed, and so how additional water molecules bond to them are much more arbitrary. That's why these snowflakes are kept small and the microscope is so powerful: they're more identical when they're less complex.

Source: Forbes January 14, 2017 15:19 UTC