Essential oils are produced in the cells of aromatic plants and are held in specialized glands. They are released from the plant and collected (concentrated) most often through steam distillation (and sometimes hydro or water distillation or a combination thereof). Distillation is a method of separating components based on differences in volatile constituents in a heated mixture. Steam distillation involves bubbling steam through the plant material. The temperature of steam is easy to control, making it ideal for heat-sensitive essential oils. The essential oils contained in plants are immiscible in water and have a higher boiling point, allowing the essential oil to vaporize at a lower temperature than it normally would on its own. Other methods used to create pure essential oils are dry or vacuum distillation, dry/destructive distillation, and expression (for citrus peels). Expression, also referred to as "cold pressed", is a method where oil is obtained by using high mechanical pressure to literally squeeze the oil from the plant material.