The time evolution of the system is described by a map formula_11. Given some subset formula_6, its map formula_13 will in general be a deformed version of formula_14 – it is squashed or stretched, folded or cut into pieces. Mathematical examples include the baker's map and the horseshoe map, both inspired by bread-making. The set formula_13 must have the same volume as formula_14; the squashing/stretching does not alter the volume of the space, only its distribution. Such a system is "measure-preserving" (area-preserving, volume-preserving).