To crystallise is to form crystals — either by letting a substance arrange itself into an ordered lattice, or by deliberately coating or impregnating something with crystals. In the kitchen the word covers two different operations. The first is the physical change: sugar crystallising out of a supersaturated syrup, honey setting into a grainy mass, or cocoa butter adopting a new crystal form during tempering. The second is a preservation and decoration technique: fruit peel, flowers, or ginger are crystallised by soaking them in strong sugar syrup until sugar penetrates the tissue and then drying them so a sparkling crust of crystals forms on the surface. Both senses turn on the same chemistry — sugar solutions driven past saturation — and both demand control of temperature, concentration, and seeding to get the desired crystal size.