open end of a water balloon, it is possible to use it as a makeshift water gun. Solar lift Solar balloons are thin, large balloons filled with air that is heated by the sun in order to decrease its density to obtain lift. Rockets Balloons are often deliberately released, creating a so-called balloon rocket. Balloon rockets work because the elastic balloons contract on the air within them, and so when the mouth of the balloon is opened, the gas within the balloon is expelled out, and due to Newton's third law of motion, the balloon is propelled forward. This is the same way that a rocket works. Flying machines Balloons filled with hot air or a buoyant gas have been used as flying machines since the 18th century. The earliest flights were made with hot air balloons using air heated with a flame, or hydrogen as the lifting gas. Later, coal gas and later still helium were used. An unpowered balloon travels with the wind. A balloon which has an engine to propel it is called a dirigible balloon or airship. Medicine Angioplasty is a surgical procedure in which very small balloons are inserted into blocked or partially blocked blood vessels near the heart. Once in place, the balloon is inflated to clear or compress arterial plaque, and to stretch the walls of the vessel, thus preventing myocardial infarction. A small stent can be inserted at the angioplasty site to keep the vessel open after the balloon's removal. Balloon catheters are catheters that have balloons at their tip to keep them from slipping out. For example, the balloon of a Foley catheter is inflated when the catheter is inserted into the urinary bladder and secures its position. Insertion of balloons subsequently filled with air or liquid can be used to stop bleeding in hollow internal organs such as stomach or uterus. History Humans have intentionally filled bladders, especially actual animal bladders, with air since prehistory. In Ancient Greece, these had a number of recorded uses. The Aztecs inflated cat intestines to make shapes to present as sacrifices to the gods. By the 18th century, people were inflating balloons of cloth or canvas with hot air and sending it aloft, the Montgolfier brothers going so far as to experiment with first animals in 1782, and then, when altitude did not kill them, human beings in 1783. The first hydrogen-filled gas balloon