The 17th-century astronomer Johannes Kepler was the first to muse about the structure of snowflakes. Why are they so symmetrical? How does one side know how long the opposite side has grown? Kepler thought it was all down to what we would now call a “morphogenic field” – that things want to have the form they have. Science has since discounted this idea. But the question of why snowflakes and similar structures are so symmetrical is nevertheless not entirely understood.