Balloons are usually filled with gases such as hydrogen, helium, nitrous oxide or air. The floating of a balloon is based Archemedies principle. If the weight of an object is lesser than the weight of the gas or liquid that it displaces, the object will float. This principle can thus be applied to balloons filled with gasses other than air. Earlier, balloons were filled with hydrogen to make them float. Nowadays, it is mostly Helium that is used in rubber balloons. Large hot air balloons have also been used to fly. Depending on the size, a balloon can keep floating for 18 hours to even several days. Some tips to increase the flying time of a helium balloon is filling it fully, without twisting it too much and cutting off the rolled nozzle after tying it. Sometimes special gels are used to coat the inside of the balllon which reduces helium leakage and thus increases the float time of a balloon. If filling your own balloons you can even use a disposable helium cylinder, which usually cost somewhere between £25 and £29.