A balloon is a flexible bag fille with a type of gas, such as helium, hydrogen, nitrous oxide or air. Early balloons were made of dried animal bladders. Modern balloons can be made from materials such as rubber, latex, polychloroprene or a nylon fabric. Some balloons are purely decorative, while others are used for specific purposes such as meteorology, medical treatment, military defense, or transportation. A balloon's properties, including its low density and relatively low cost, have led to a wide range of applications.