Christner collected large samples from 19 fresh snowfalls in regions as diverse as Montana, France, and Antarctica, and isolated the ice nucleators by filtering the melted snow. Surprisingly, he found traces of cells containing DNA among the nucleators in every single site. Both heat, which deforms the ice-nucleating protein, and lysozyme, an enzyme that wrecks a bacterium’s outer wall, robbed the vast majority of these particles of their ability to form ice.