At zero temperature or absolute zero, the molecules of a substance have the lowest possible energy and are in their most ordered state. This state is known as a Bose-Einstein condensate (BEC). In a BEC, the molecules move in a coherent manner and the substance exhibits macroscopic quantum effects. The volume of a substance in a BEC is not zero, but it is very close to zero. The density of a substance in a BEC is very high, and the molecules are packed together very tightly.