A water balloon or water bomb is a balloon, often made of latex rubber, filled with water. Water balloons are used in a summer pastime of cooling off through water balloon fights. Water balloons are also popular for celebrations, including celebrating Holi and Carnival in India, Nepal, and several other countries. A water balloon (also known as a water bomb) is a latex rubber balloon filled with water used in water balloon fights, during some festivities, and as a practical joke. Simple as it is, a water balloon is actually an impressively efficient energy storage medium. The efficiency is 85–90% when a water balloon stores and releases energy at room temperature. Traditional water balloons are made from latex, rubber, nylon, and polychloroprene materials, all of which do not readily biodegrade. It takes over four years for a rubber or latex water balloon to break down. Single-use water balloons are made of latex which comes from the sap of the rubber tree. To make this natural product into an elastic, rubbery, coloured balloon lots of chemicals need to be added including sulphur, heavy metals, plasticizers and pigments. The end result is a long way from the original natural latex. Warm water is less dense than cold water, so the cold water in the water balloon was more dense than the hot water and so sank. As the water cooled down, it’s density increased making the balloon float again. The water balloon has retained much of its form and use since its first appearance in 1950. Back then, British inventor Edgar Ellington was merely looking for a way to treat trench foot by filling up a latex sock with water. Seeing that it leaked, he threw the watery orb against a table and watched it explode.