A balloon is a flexible bag normally filled with air or gas. Early balloons were made of dried animal bladders.
Balloons as flying machines
Large balloons filled with hot air or buoyant gas have been used as flying machines since the 18th century.
Balloons as decoration or entertainment
Party balloons are mostly made of artificial polymer rubber and can be filled with air, helium, water, or any other suitable liquid or gas. The rubber makes the volume adjustable. If the balloon is blown up too much the balloon bursts with a bang. Rubbing a balloon produces a special sound that is very unpleasant for some people.
Filling with air is done with the mouth or with a pump.
When rubber balloons are filled with helium so that they float (restrained by ribbons or strings) they can hold their shape for only a few hours. The enclosed air or helium escapes through small pores, and helium atoms being much smaller than the nitrogen and oxygen molecules in air, it escapes much quicker. Even a perfect rubber membrane eventually loses helium to the outside, and its contents are contaminated by oxygen and nitrogen migrating inward from the outside. The gases in question actually dissolve in the rubber on one side and are released from solution on the other. The process by which a substance migrates from a region of high concentration, through a barrier to a region of lower concentration is called osmosis. The inside of balloons can be treated with a special spray (e.g. "Hi Float" brand) to reduce the helium leakage.
Beginning in the early 1990s, some more expensive (and longer-lasting) helium balloons have been made of thin, unstretchable, impermeable Mylar films. These mylar balloons have attractive shiny reflective surfaces and are often printed with color pictures and patterns. The most important attributes of Mylar for balloons are its light weight, increasing buoyancy and its ability to keep the helium gas from escaping for several weeks. However, there has been some environmental concern, since the Mylar does not biodegrade or shred as a rubber balloon does, and a helium balloon released into the atmosphere can travel a long way before finally bursting or deflating.
Partygoers sometimes entertain each other by untying a balloon and inhaling the helium. Because the speed of sound in helium is about twice that in air, the helium causes the vocal tract to become more responsive to high-pitched sounds and less responsive to lower ones. The result is a voice that sounds high-pitched (and usually very funny).
Balloon artists are entertainers who twist and tie inflated tubular balloons into sculptures (see balloon animal). The balloons used for balloon sculpture are made of extra-stretchy rubber so that they can be twisted and tied without bursting. Since the pressure required to inflate a balloon is inversely proportional to the diameter of the balloon, these tiny tubular balloons are extremely hard to inflate initially. A pump is usually used to inflate these balloons.
Decorators may use dozens of helium balloons to create balloon sculpture. Usually the round shape of the balloon restricts these to simple arches or walls, but on occasion more ambitious "sculptures" have been attempted.
Water balloons are thin, small rubber balloons intended to be easily broken. They are usually used by children, who throw them at each other, trying to get each other wet. (See practical joke.) A popular game with this idea is a water balloon toss, where two lines of people stand opposite each other and throw balloons back and forth. This continues until the balloons burst, showering whomever failed to catch it.
Balloons in medicine
Angioplasty is a surgical procedure in which very small balloons made of a special material are inserted into blocked or partially blocked blood vessels near the heart. Once in place, the balloon can be inflated to clear or compress arterial plaque, and to stretch the walls of the vein. A small stent can be inserted in its place to keep the vessel open after the balloon's removal.